Sounds like it's picking up some digital crap from somewhere. Tubes going microphonic?
+1. My guitarist uses a Mashall TSL 100 watt 2x12 combo (phwoooor!) and it started to do the same thing, then it blew a fuse, then died.
I had a look at the power tubes and one of the four was showing a blueish glow. Cheapo tubes, out, matched JJ's, in, sings much sweeter now.
Looks like I need some new tubes then. Magic.