This has only happeded three maybe four times.
When im tootling along in 5th gear at about 2500 3000 revs on say a motorway sometimes I get TSSSSSCHHHHHHHHH from the engine bay like somethings going faster than the revs
It sounds like (and I dont know) but if I was to guess the turbo spinning and not releasing like its spinning to max and not releasing. If I press the clutch in or drop below 2000 revs this normally then stops. Incidently my turbo kicks in at 2000 which is why I think it may be that and ive never had a non aspirated engine make this noise.
Dont get me wrong it only does it a few times its not like every day but its is concerning.