I have a question, and I realize this is a pretty old thread but its the best one I've found that sounds like the same issue I'm having. I have a '04 manual, and I've been experiencing the same high pitch humming sound and it seems to slowly be getting worse. I didn't hear/notice it at all when I bought the car in December. It currently has 84K miles on it.

My question is, are there differentials in the front as well? Because the OP's explanation describes my sound perfectly only it seems to come from the front, not the rear. If it is the differentials, is this a major problem that needs to be fixed ASAP or is it merely an annoying noise? I bought my car used so it does not have a warranty, sounds like this is a big/expensive repair?

Thanks for any help