Audi A5 Convertbile 2.0 tdi s-line 59 plate 17k miles loud screeching sound

[Audi Dynamik]

Hello, i was casually driving my Audi A5 Convertbile 2.0 tdi s-line 59 plate 17k miles which has been serviced not so long ago down the road and i heard a terrible high pitch squealing sound , it went on for about 10 minutes and it was coming from my car, but my car was driving perfectly normally. it inst an issue but i am very curios of what it where maybe a stone stuck in the wheel or brakes or something ? this is now the second time it has happened and it has happened on my previous a4 convertible ?
has anybody else experienced this problem and know what it is ?
Thanks

 

[s3 performance]

stone stuck in brakes i think mate