This is a well documented problem that occurs when you start installing extra drives that vibrate.Just be glad that yours isn't doing the harmonic resonance thing... do a search for resonance and MacPro and you'll know what I'm talking about. When I installed my new Western Digital drives, I wound up with a sound that reminded me of an airplane flying in the distance. The buzz would get louder and then start to go away, then come back again. It was driving me crazy! What I wound up doing was buying some hard density foam rubber and cutting it into squares. I jammed those squares into the spaces between the drives and the mount. I also added washer and rubber O-rings and that seemed to help. Now that my machine is several months old and I added the sound proofing, I rarely get the humming noise. Every once in a while, I have to tug on the handle or knock on the side of the case and the sound goes away.