This morning, I picked up my PB (12" Rev. A) from the breakfast table after having my coffee and carried over to my study (still turned on), when I noticed that it makes a funny humming noise when I tilt it. The humming occurs when I am actually changing the angle of tilt (e.g. going from horizontal to the 60 degree angle necessary to put it on the stand on my desk, or when it tilts a little when I'm walking). However, it stops as soon as I stop changing the angle of tilt (e.g. if I leave it at a 60 degree angle, undisturbed). It does not appear to have any relationship to when the hard drive is accessed -- just when I tilt the computer. I checked the status of my hard drive in Disk Utility, and it says the hard drive's SMART status is verified. Also, the hard drive doesn't make the unusual clicking/grinding noises that one would normally associate with hard disk failure. But I'm backing up the hard drive right now, just to be safe. Any thoughts on what could be causing the humming noise?