Kind of a chirpy, bird like squeak when you shift the rear? That could be the jockey wheels (the little wheels on the rear derailleur) - they have a bearing in the middle that might need some lubrication, or they may need to be replaced if the hole that the bearing fits into is no longer round. They are pretty cheap and easy to fix though. The parts costs about $15 and if you are a little handy you can replace them yourself.