Speaker Hum on my Quad 12L Active's


I have a simple set up in my home office and up until recently things have been fine, but recently my Quad's have developed this hum. They hum when I'm not playing any music, plus I test them and they hum when the RCA cables are connected to the speakers, but not to anything else. Not sure what that means.

I usually play music from my computer wirelessly to my Airport Express and then go from there with RCA's to my active speakers. Any thoughts?

Also, I've checked around the forums for this topic and none of the previous questions seemed to fit my situation. Would appreciate any expertise here.
bdogsquad
Are the power cords plugged into the same circuit? To test, use an extension cord and plug them into the same outlet.
It doesn't sound like the speakers because both started at the
same time. Try the interconnects into a preamp,or even a walk man type cd player,any source with a volume control. Try it with the new source unplugged from the outlet too.That might show a ground loop problem. Besides the above mentioned,I don't have any other ideas.
When you say power cords, you're talking about the individual cords for the two speakers right? Currently, I have the Airport Express, the two speakers, a preamp and a turntable all plugged into a powerstrip in that's plugged into a wall socket across the room from where my computer is.
I think that's what he had in mind?If the plugs aren't polarized,try reversing one of them too.Just another thought.
odd question, but do you have a dimmer switch somewhere in the room? if so, look there as a potential issue.

just a thought.