Help identifying a ground loop hum

Ok, I'm talking basic--I now wouldn't know one kind of humm from another if it bit me in the face. I'll describe what I've got, someone please tell me what it is... About 50% of the time that the pre-amp is on, there is an audible, electronic hum that comes directly from its power supply. The power supply hums--not all of the time, sometimes at first, sometimes only later, sometimes not at all--but it defintely hums. Is this a ground loop hum? Is it just sporadically noisey, and them's the breaks? Is there a problem with the pre-amp? Many thanks.