Hi, This is a copy of my post on the S3 forum, which may be of help/interest to other turbocharged car owners: This is by way of a summary of my searches for an answer to these noises. They came about on start up, when the engine made chirping noises, which disappeared after a few seconds. It was possible to avoid the noises by driving gently, but stop the engine and fire up again and the chirps return. I was not the only poor sod seeking an answer to this and our various efforts at a resolution are recorded in various S3 threads including BOOST DIAGRAM and CAN THIS BE A SLIPPING CLUTCH. Trawling through other VAG forums for turbocharged engines, I came across various posts/threads on engine noises variously described, but very much like "chirping" that I was trying to resolve, but none provided a reason for the noise neither was an answer to the problem mentioned. My efforts can be briefly summarised thus: A slipping clutch necessitated the removal of the gearbox, and with this out access to the turbo was very much easier. Manual inspection indicated obvious axial play in the impeller shaft and a new KO4 was obtained and fitted. All the associated plumbing for air, water etc was re-used but a new oil supply pipe was fitted. I enquired about rebuilt KO4's but came away from the idea, simply because there was no broad based opinion on how effective a rebuild would be and how long it would last. Didn't want to do this work again. So, dual mass flywheel, clutch and turbo all replaced and on start up: No chirping. What's more, the car's performance is in a different class - noticeably more grunt. So for me "chirps" etc on start up equaled turbocharger replacement. I still have the noisy unit and when work allows I will dismantle it and see what's what but it is evident that the bearings have failed and the oil pressure that would normally allow the shaft to "float" has not been retained in the bearings. Oil mist in the air ducting was evident and this may provide a clue as to whether owners of cars with similar noises, in fact have the same problem as I did. What is a bit of an eye opener is that this is a genuine 48k 2001 S3 with FASH. Clutch replacment was due to a leaking gearbox oil seal, the dual mass flywheel seems to be an inherent weakness and the turbo? Who knows. Hope this helps "chirpers", everywhere and good luck.