B8.5 CREC - very faint knock/rumble at idle - possibly from DSG?

Jake123

New Member
Hello All,

Got a 64 plate b8.5, CREC engine, 33k miles with full history.
Car is new to me

Noticing a faint knock/rumble at idle with car sat in park, goes as soon as revs are raised.
Sounds like it's from rear lower, possibly gearbox area (can hear through drivers side wheel well most prominent). With my ear to the grill/rad area it isn't so audible

Is this a usual DSG sound maybe or are they meant to be silent?
Drives perfect with zero issues -, just noticed it when got out and left it ticking over!

Tried to find a few idle videos but most have noisy exhausts or are showing chain rattle - this isn't a rattle sound it's quite deep and remains fairly constant even once warmed up.
Obviously you can't hear it inside car or once moving, only when idling and you listen for it.
Not as loud as a bottom end or rod knock type noise

Thanks in advance for any input
 

Jake123

New Member
Car is bone stock.

I'm mid Bedfordshire - will try to get an in person opinion or listen to another.

It's very faint and I'm sure most people would not even notice!
 

ninjag

Registered User
I've got a stock 2015 S4 Avant with the CREC engine with 66k miles and I've not heard this noise and I get a lot of slagging for being so sensitive to unusual car noises! lol

Does it happen all the time or only when the car is cold/warm? I'll have a closer listen to mine to see if I can hear anything. Might be worth getting yours up on a ramp or lift so you can get underneath for a closer listen.
 

Jake123

New Member
Thanks for the reply!

It seems pretty constant yes, cold or warm it can be heard don't think it changes much.

It's nowhere near as pronounced as the timing chain type noises people get and is quite a deep noise, if that makes sense?

I could try to video it but it's so hard to accurately capture sounds over camera!

Thanks for input!
 

ninjag

Registered User
I think this may be a discussion of a similar noise - https://www.audiworld.com/forums/a6...on-194/3-0tfsi-engine-knocking-3004586/page2/

I need to have a listen from above and below, maybe the noise is higher up than I originally thought...

I've had a good listen to mine and there's no unusual noises that I can make out.

Regarding the supercharger, I have had my belts changed recently so I'm not sure if that could have made any difference. However, it might be worth taking into account that with the CREC engine the supercharger has a clutch which I believe disengages whilst coasting and also at idle in order to save fuel. So would the supercharger itself not be inactive whilst idling? I'm guessing the pulley section would still be in operation, maybe it is something related to this and it's making a noise when not under load or tension or something? Just throwing some ideas out there with my limited technical knowledge! lol
 

Jake123

New Member
I've had a good listen to mine and there's no unusual noises that I can make out.

Regarding the supercharger, I have had my belts changed recently so I'm not sure if that could have made any difference. However, it might be worth taking into account that with the CREC engine the supercharger has a clutch which I believe disengages whilst coasting and also at idle in order to save fuel. So would the supercharger itself not be inactive whilst idling? I'm guessing the pulley section would still be in operation, maybe it is something related to this and it's making a noise when not under load or tension or something? Just throwing some ideas out there with my limited technical knowledge! lol
Thanks for your input and taking the time to listen, appreciate it.

I could pop the supercharger belt off and it would isolate that, not sure if that would create any codes though or any issue running car with that belt removed?
It also seems more rear of engine/firewall area, but could be echoing.

Might get it booked in with Audi as it needs the DSG oil doing soon and is under extended warranty, be interesting to see if they are willing to investigate/cover it... we shall see!

I'll update if I do get any further info or a resolution, might be a few weeks though.
 

ninjag

Registered User
Thanks for your input and taking the time to listen, appreciate it.

I could pop the supercharger belt off and it would isolate that, not sure if that would create any codes though or any issue running car with that belt removed?
It also seems more rear of engine/firewall area, but could be echoing.

Might get it booked in with Audi as it needs the DSG oil doing soon and is under extended warranty, be interesting to see if they are willing to investigate/cover it... we shall see!

I'll update if I do get any further info or a resolution, might be a few weeks though.

It wouldn't do any harm to get the DSG fluid done, it's always struck me as a little strange why there's no time limit on it and only mileage. But with the fluid being over 7 years old who knows. I do a lot of short journeys which I believe manufacturers consider to be 'harsh' conditions so I tend to change my fluids more often.

There's also the hypoid gear oil for the front diff, gears and transfer case which may be worth doing if you are planning to keep the car, although your mileage is low but then again 7+ years... same for the rear diff. I've got both of these on my to-do list either this year or next. I'm still try to learn as much as I can about this car.
 

Dippy

Registered User
Because the clutch is magnetic the supercharger rotor is always turning - at idle around 200-300 rpm. I would doubt that it is the supercharger if the noise is anything like that video you linked to.

I would have thought I'd have noticed if mine was making a similar sound but I'll try to remember to listen for it.
 

ninjag

Registered User
Because the clutch is magnetic the supercharger rotor is always turning - at idle around 200-300 rpm. I would doubt that it is the supercharger if the noise is anything like that video you linked to.

I would have thought I'd have noticed if mine was making a similar sound but I'll try to remember to listen for it.

So is there still some sort of connection due to the magnetic nature of it or is the magnetic aspect fully deactivated? I would have thought it was more that there would still be some rotation because the incoming air is still naturally flowing through the rotors.
 

ninjag

Registered User
Digressing slightly, but an interesting diagram regarding the s/c behaviour. I'm assuming if you whack the car into manual mode and keep the revs above 3000rpm regardless of load/torque demand then you can be assured that the supercharger will remain engaged, but below 3000rpm then it depends on the other parameters?

1642600970446.png
 

Dippy

Registered User
The clutch is held closed in manual and sports mode
 

Dippy

Registered User
That's good to know thanks. I'm sure this has come up before and you weren't sure re. sports mode? It makes sense to be engaged though.
That's because I rarely use sports mode directly - it is usually as part of ADS mode. But I did check it not so long ago so I think that it's only the gearbox mode that affects clutch control, and not ADS itself.
Is this true even when sat at idle in park?
It certainly is true of when in idle, but I can't 100% say that it is when in park.
 
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