My Disco had the same anoying squeek, so i put up with it! (DONT)
The noise seem to come from the top pully wheel. It eventualy stripped the first 5 mm off the belt and that flapped round and ripped the sound proofing off the inside of the bonet. I had the belt changed afterthat, but it only lasts 7 months before it starts again.
Pully wheels didnt seem to be loose, so i just keep an eye on it now!