My Logitech Z313 speakers constantly emit a low audible hum. It's constant and not relative to the volume level. There is no noise if I connect headphones to the computer and it doesn't go away in the speakers when I disconnect them from the computer, so it's not coming from the audio line out, nor does it seem to be a ground-loop.
It's not cell-phone RF interference since that shows up very noticeably when my cell communicates with the tower and is very distinct.
My router and wireless internet modem are a few feet away (above) the speakers and about a foot more above the amp/subwoofer.