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.