have read old forums/posts as I too had a persistant squeak (like chirping sound) from the engine of my corsa - no specific time it made this noise but went on and off with and without acceleration etc. After it went on for a while a local garage advised it was the serpentine belt and so this was changed as well as the tensioner but it still squeaked! The garage said it needed time to "bed in" or it could possibly be the air conditioning pump, the water pump and the list went on and on! Not wanting to throw good money after bad (and wasn't convinced about the "bedding in" comment) so took it to another mechanic who checked over and couldn't tell me either although he took the belt off and said there was still a squeak so it wasn't any of the pumps the other garage listed. Also there wouldn't have been anyhting wrong with the belt! Drove for a wee while longer and then there was another "fluffing " type noise started- took it straight back to mechanic who immediately diagnosed it as a faulty fuel injector - checking this out and it was the case (actually two had gone) and so he put back refurbished fuel injectors and washers etc and hey presto - no more sqeak!! The sqeak noise had been the air pushing through the knackered fuel injector and then when it worsened, the second noise started. This real mechanic saved me 100's of pounds - just thought I would share this as a lot of the other old posts went through the same as I did (but spending money unnecessarily) which is all very frustrating.

I would have attached this comment on to past posts but can't see how

Hope I help at least one other get to the bottom of their problem...