check all shielding, you can buy sticky-back metal foil, that seems to work pretty well for your cavities,
also try using another amp, as pauseflash says, it could just be the noise from your amp, do you play it loud? if you have it turned up loud, when you arnt playing, the amp will just be amplifying its own internal noise. also it could be mains hum, again from the amp. maybe try another mains socket somewhere else in the house.
hope this helps,
will