Buzzing noise

Hey folks!

Recording vocals using GB2 on my G4 eMac has been a pain because I have to use a bunch of filters to try and eliminate the constant buzzing noise that appears in every vocal track I record. It makes for a very non-natural sound. I have a Shure mic on my M-Audio mobilePre, so I thought everything would be golden, but even after resetting the P-RAM a hundred times, there's still no change.

Any suggestions? Thanks.