Roland Cube 60 - Fuzzy Buzz

[Ted]

Got an old Roland Cube 60 here. Generates a kind of kind of fuzzy buzz on the lowest F#/G/G# on E and D strings of a guitar. Nowhere else. Its not the speaker. I note that these frequencies are around 50 and 100 Hz, but that may be a red herring. Nothing visible on the scope into a dummy load from a sig gen at those frequencies.

Any ideas?

[Ted]

Now here's a strange thing. Ran it up to full power at 1kHz into a dummy load.
The 500W amp in the next room - with nothing connected to its input - started emitting a 1 kHz tone. Mains borne I guess.

I'm thinking dodgy power supply electrolytics