Thank you all, gents, for your helpful suggestions. Tom Cavanaugh just about hit the nail on the head. It's coming from the L/Mono output of my harmonizer which is powered by a "12vDC mains adaptor". So, short of buying a harmonizer powered direct from the mains, I guess I'm stuck with rf for the time being.

There are no radio station transmitters in the immediate area, the nearest being about 8 miles away. I suppose that's close enough, though.

Thanks again for your help, guys.