Bit of a long shot but if something electrical is radiating you can sometimes hear it on an AM radio tuned off a station, you can try tuning it up and down if that helps. If you can pick it up you may get a clue of the source by moving the radio about.
Good idea to disconnect the speakers, if it is interference it must get into something that can make a noise and the speaker/amps are the prime suspect.
Another thing to try is an audio spectrum analyser, I use the Spectroid app on an Android phone and it's not bad at measuring and displaying a chart of audio spectrum. If you can measure the main frequency you may be able to relate it to engine revs. I'm as deaf as a post so I find the app useful for fault-finding sounds I can't hear.