Given the results Ryszard has had, and the descriptions he's given, I'm inclined to agree that there's something within the PC that's the real cause.

The fact that the noise is apparently 8 times the mains frequency points to some kind of filtering issue as clearly there is some kind of unintended "signal processing" going on.

A temporary ground lift on the PC with a "2 prong" adaptor is a good test. It will at least prove whether breaking the earth loop somewhere will alleviate the problem.

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!