B8 S4 Avant Constant Rear ‘Hum’ Issue

[S444AVT]

Hi,

I have a 2010 S4 B8 Avant with the Rear Sports Diff and Drive Select extras. One owner from new (prior to me acquiring the car in January 2022). Full Audi service history, 72,000 miles.
Last week on a motorway run, a ‘hum’ developed that sounded exactly like a wheel bearing had gone. It is constant so no let-up on cornering, and it sounds like it is coming from the rear passenger side.
Is there any way to check if this is a wheel bearing Vs a rear-diff issue without changing the bearings and hoping for the best?
.At around £400 per bearing with labour/parts this seems like a costly gamble.
So is an equally costly Audi diagnostic the way to go?
Any advice greatly appreciated.
Cheers
Lee

 

[NHN]

Did you swap wheels around yet?

 

[S444AVT]

Not heard of this… is this swapping back to front to see what change (if any) occurs?

 

[NHN]

Yeap, if the tyres aren't directional, swap front to back, side to side, see if it changes.

 

[S444AVT]

Okay cool. Thanks for the advice - any change will highlight the likely wheel bearing.
No change = probably differential related.
Brilliant, thanks!

 

[abmat]

I had a hum and it was the tyres.

jack the car up and see if there is any play in the wheel top to bottom.

 

[rum4mo]

Modern wheel bearing are not too helpful with handing out easy clues like "you can rock the wheel", I had that at the front of my old 2000 B5 Passat 4Motion first noted when it had its directional winter tyres on, I swopped to summers - no change, so with a lot of doom and gloom booked it into my local VW dealership and said if it is wheel bearings replace both sides, although considering that it was diff trouble - it turned out to be wheel bearings, so that was a relief.

Edit:- on the other hand, my wife's 2015 VW Polo 1.2TSI is making these noises with its winter tyres on, so soon I'll be able to work out if fitting the summer tyres sorts that out, and if it does, I'll be getting a full four wheel alignment check carried out as the directional winter tyres feel a bit "saw toothish".

 

[S444AVT]

Not heard of this… is this swapping back to front to see what change (if any) occurs?

Yeap, if the tyres aren't directional, swap front to back, side to side, see if it changes.

I think I misunderstood yesterday.
Changing the wheels around will only highlight if it is a tyre issue, not a bearing issue.
The wheel bearings will remain within the hub in each corner unaffected by the wheel position change..
Tyres checked, all above 7mm depth and all even wear across the surface. Similar depths front to rear too, but will swap over just to be sure.
So if I rule out tyres then I’ll need to have the diff diagnosed as there is no play or noise from the bearings when turning by hand on the ramp

 

[S444AVT]

I had a hum and it was the tyres.

jack the car up and see if there is any play in the wheel top to bottom.

Thanks…. No play at all, non-directional, even wear across tread surface, all above 7mm depth front and rear. Hoping it’s bearings and not the diff

 

[S444AVT]

Modern wheel bearing are not too helpful with handing out easy clues like "you can rock the wheel", I had that at the front of my old 2000 B5 Passat 4Motion first noted when it had its directional winter tyres on, I swopped to summers - no change, so with a lot of doom and gloom booked it into my local VW dealership and said if it is wheel bearings replace both sides, although considering that it was diff trouble - it turned out to be wheel bearings, so that was a relief.

Edit:- on the other hand, my wife's 2015 VW Polo 1.2TSI is making these noises with its winter tyres on, so soon I'll be able to work out if fitting the summer tyres sorts that out, and if it does, I'll be getting a full four wheel alignment check carried out as the directional winter tyres feel a bit "saw toothish".

I’m hoping for the same diagnosis as non-directional summer tyres all good on inspection

 

[ninjag]

You could spin the wheels and feel the springs for vibration to help rule out wheel bearings.

 

[S444AVT]

You could spin the wheels and feel the springs for vibration to help rule out wheel bearings.

Cheers, but with the Quattro system the drivetrain and some linkages need to be uncoupled to spin the hubs independently to stand a chance of hearing/feeling something isolated… and that’s the route I’m going with an Audi specialist near me.
My fear is I’ve made a bit of a schoolboy error in not changing the diff and gearbox oil when having a stage 1 ECU/TCU remap. With hindsight I should have asked for advice on what precautions to make before upping the power to 461 BHP. Consequently I think I’ve actually given myself a bit of an expensive fix in my greed for more power

 

[s4chopper]

Cheers, but with the Quattro system the drivetrain and some linkages need to be uncoupled to spin the hubs independently to stand a chance of hearing/feeling something isolated… and that’s the route I’m going with an Audi specialist near me.
My fear is I’ve made a bit of a schoolboy error in not changing the diff and gearbox oil when having a stage 1 ECU/TCU remap. With hindsight I should have asked for advice on what precautions to make before upping the power to 461 BHP. Consequently I think I’ve actually given myself a bit of an expensive fix in my greed for more power

Not recommended but myself I’ll usually jack up securely on 4 axle stands and have an accomplice drive the car up into second or third gear. Then I’ll get under the car and listen out, if it’s not obvious I’ll drop a stethoscope to listen for rumbling on the hub. Even then a bad wheel bearing can’t be heard if it’s not under load.

Rear diffs rarely go but not impossible, Audi class them as sealed units, and you can’t get any of the 4 SKF bearings inside which are two on the pinion shaft and one either side on the output flanges. I have rebuilt many of these rear diffs and use Timken equivalents to good effect.

I wouldn’t be paranoid about not replacing diff oils whilst upping power, it wouldn’t contribute to a failure in isolation.

 

[S444AVT]

Not recommended but myself I’ll usually jack up securely on 4 axle stands and have an accomplice drive the car up into second or third gear. Then I’ll get under the car and listen out, if it’s not obvious I’ll drop a stethoscope to listen for rumbling on the hub. Even then a bad wheel bearing can’t be heard if it’s not under load.

Rear diffs rarely go but not impossible, Audi class them as sealed units, and you can’t get any of the 4 SKF bearings inside which are two on the pinion shaft and one either side on the output flanges. I have rebuilt many of these rear diffs and use Timken equivalents to good effect.

I wouldn’t be paranoid about not replacing diff oils whilst upping power, it wouldn’t contribute to a failure in isolation.

Thanks for the tips, and a degree of comfort that diff oil changes are not necessarily essential when increasing output.
I’ve got an Audi specialist near me that runs his own S2 and RS6 so should be clued up and reasonable labour to help me. Maybe run it and listen from underneath before going to the lengths of deactivating the Quattro drive to turn each hub by hand.

If no play/rumbling detected from the hubs…..and therefore could be ‘Diff related’ is there anything to be done to establish this?

Or is it a case of replace both rear bearings with fingers crossed, and if the noise persists, it’s most likely the Diff? Or do you go straight for further investigation on the diff at that point?
Cheers

 

[quattrogone]

when my rear diff went on a b6 quatty it sounded like I was driving over cats eyes at low speeds or under load pulling away.
To start with it was barely audible but got progressively worse and really bad as soon as I sold it...(I did make a contribution to the repair)

 

[S444AVT]

when my rear diff went on a b6 quatty it sounded like I was driving over cats eyes at low speeds or under load pulling away.
To start with it was barely audible but got progressively worse and really bad as soon as I sold it...(I did make a contribution to the repair)

The noise is definitely a whir/hum that came on with no build up or worsening. It is audible as soon as you start moving, and speeds up (gets louder) as the vehicle speeds up and slows down on deceleration… it’s pointing more towards a bearing I think

 

[rum4mo]

With my 2000 VW Passat 4Motion (A4 B5 quattro in cheaper/newer clothes), had front wheel bearing problems it sounded a lot like the noise army trucks make with their all terrain tyres one, ie "wah wah".

 

[S444AVT]

That’s the noise
Attacking the job this week as the wah wah is louder than the valved exhaust at 6500rpm

 

[s4chopper]

Thanks for the tips, and a degree of comfort that diff oil changes are not necessarily essential when increasing output.
I’ve got an Audi specialist near me that runs his own S2 and RS6 so should be clued up and reasonable labour to help me. Maybe run it and listen from underneath before going to the lengths of deactivating the Quattro drive to turn each hub by hand.

If no play/rumbling detected from the hubs…..and therefore could be ‘Diff related’ is there anything to be done to establish this?

Or is it a case of replace both rear bearings with fingers crossed, and if the noise persists, it’s most likely the Diff? Or do you go straight for further investigation on the diff at that point?
Cheers

Diagnose and fix what’s needed, unless you like spending money for the sake of it.
Rear wheel bearings are okay to replace, stuck ones take some persuasion. ABS sensors are usually always stuck.
If your unlucky and it is the diff at fault, ascertain the model code and final drive ratio before sourcing a working unit. Plenty available on eBay since they are interchageable across various models.

 

[S444AVT]

Thanks @s4chopper
The noise has fortunately got worse…. And that’s enabled me to hear a conclusive degree of difference when under load or off load when cornering, so now know the bearing to replace. And definitely not the diff ‍
Cheers

 

[s4chopper]

Thanks @s4chopper
The noise has fortunately got worse…. And that’s enabled me to hear a conclusive degree of difference when under load or off load when cornering, so now know the bearing to replace. And definitely not the diff ‍
Cheers

Great news. Don’t scrimp on bearing quality, SKF only. Although I bought 4 recently, ECP stock them and I got them for 50 each over the counter.

 

[S444AVT]

Great news. Don’t scrimp on bearing quality, SKF only. Although I bought 4 recently, ECP stock them and I got them for 50 each over the counter.

Yep, no point scrimping I completely agree. I’ve always had the mindset of equal as a minimum, or upgrade when replacements are required

 

[g00se]

Could this be tyre saw-toothing? I've had this before on my old B7 S4.. they had loads of tread on them, balance was good, pressures were good.. but the tread blocks were trashed

See here: https://www.continental-tires.com.gh/car/technology/tyre-knowledge/lex-1-3-1

cheers
goose

 

[rum4mo]

Could this be tyre saw-toothing? I've had this before on my old B7 S4.. they had loads of tread on them, balance was good, pressures were good.. but the tread blocks were trashed

See here: https://www.continental-tires.com.gh/car/technology/tyre-knowledge/lex-1-3-1

cheers
goose

I'm hoping that it is saw toothing that is the issue with my wife's 2015 Polo, changing from Winters to Summers will prove that, maybe when I get back from holiday! Saw toothing if it is that, and it has some uneven wear that hopefully a full re-alignment last week will have sorted out, is causing a "directional winter tyre" noise, but more pronounced than normal for these directional winter tyres - Alpins.

 

[g00se]

Could this be tyre saw-toothing? I've had this before on my old B7 S4.. they had loads of tread on them, balance was good, pressures were good.. but the tread blocks were trashed

See here: https://www.continental-tires.com.gh/car/technology/tyre-knowledge/lex-1-3-1

cheers
goose

I should have added.. the noise saw-toothed tyres make is just like a wheel bearing!

cheers
goose

 

[S444AVT]

I should have added.. the noise saw-toothed tyres make is just like a wheel bearing!

cheers
goose

Thanks Goose,
I’ve had a good old inspection after reading the link. No indication of saw-toothing inside or out…. I wonder if this tyre wear is a constant noise or does it change in cornering?
The sound on mine is definitely changing depending on which wheel is under load in the corners so I’m 99% sure it’s a bearing and also which side it’s located on.
Cheers for the info though, it was worth checking out
Lee