Speaker buzz for no apparent reason?

Hello,

I've moved the computer at some point, a self-built rig that's been going for 3+ years now, and after that I began having a problem. I didn't move it TO any new location, just did so after cleaning. I didn't change any wiring although I did unplug it for safety measure.

Now with the computer on I am noticing a very odd problem with the audio. The speakers are buzzing. It's usually a low-volume buzz but it's there. Suspecting the speakers, I attempted to switch them, and now I have two results to speak about:

-"new" speakers

They buzz quite noticeably, the higher the volume the louder the buzz. Note it is not a HUMM it is a BUZZ. Nothing I try to do can fix the buzz.

-old speakers

This is where it gets interesting. Depending on how the wire is sitting in the socket, like if it's forced down, the buzz fades away to near unhearable levels. The buzz in this set of speakers exhibits some strange behaveior: its peak loudness is somewhere halfway into the volume, any higher than that or lower and the buzzing fades away. The buzzing is found ONLY in one of the speakers, the left, although "deoptimizing" the connector's sitting in the audio slot will make both speakers buzz, and more loudly so.

And now the strangest bit... the buzzing appears to get louder when I hold my hand over the volume dial. The dial is on the side and can be grasped with the entire palm of the hand. When under maximum grasp, the buzzing gets very loud. Hand contact is NOT required, just its proximity to the volume dial. NO other speaker, or part of the speaker system, is able to alter buzzing like this. Also, there is a very strange way to completly kill the buzzing. Having my hand on either the computer case, the UPS, or an the exposed metal side of a USB extension cord plugged into the computer, with even minimal contact to the conductive parts, the buzzing caused by my hand's proximity disappears completly.

I have tried powering on the computer with only the audio and power cables in, no monitor, nothing, and was still unable to get rid of the buzz. I also tried rearranging the cables and replacing the power cable, or plugging the speakers into a different outlet. Nothing worked. I am hoping that the phenomena described above will provide more clues as to what kind of problem this is!

Thank you in advance.