Humming sound like a ground loop with added hard drives

Question

I get a humming low freq Hum from my MacPro from adding or added hard drives. I am using Seagates 320's ( 3 - non raid ). The MP is under my desk and I think some may not hear what I do if sitting on a desktop.
I can't figure out if it's a ground loop issue, more than one drive spinning at the same speed or some mechanical isssue, like metal to metal contact.
Just one drive with the OS. The Hum if still there is not something I am aware of or bothers me. But your room must be quiet to hear ir best.

Answer 1

As I mentioned before, some people will not hear the "hum" if the MP is sitting on your desk ( or one hard drive ) . Mine is under a large desk that is 6' long, which kinda amplifies the sound and creates a sound box for the low freq hum. Some what like adding a subwoofer. But when you add one or more drives it will Hum and I think I know the cause.
One is there is no isolation ( ie rubber ) between the hard drive mounting system, the (metal) inside frame and outer (metal) case. This causes the MP to become or act like a tuning fork.
For those who do hear the hum sound, you can take both hands on either side of the MP and "squeeze" the sides and hear the sound get lower. Take your hands off and hear it louder. So the whole MP is vibrating or creating "noise" or low freq Hum ( tuning fork) .
The problem is some what resolved with using 2 drives if you mount one in bay 1 and the second in bay 4. ( not side by side ) In other words, the sound is amplified to a higher degree if you have matching drives ( non raid ) next to eachother.
This issue will drive you nuts if you have it or can hear it. The MP stock 2.66 thats a month old is what I have. Maybe some other Macs don't show this same sound.
Whats interesting is the metal case, both outer and inter being connected as a "rigid unit" is causing or lets say, making matters worse. The other of which I have not plotted the freq, I think is around 60hz which just so happens to be the same as your " AC power". So the humming could sound like a ground loop, when in fact the sound originates from the hard drive and is being magnfied by the rigid non insulated MP that might be inhancing it's own "resonate frequency"and acting like a tuning fork.
Now if I am right and the MP is reacting as a tuning fork. What is the solution? I don't really know. And I have not tried any so called fixes other than switching bays. I have some ideas. But only those that would not cause a warrenty issue.
I hope Apple will take a look at the issue and see if a real engineer can first identify the issue and come up with a solution.