Bad cord. That's my vote because the same thing happened to me last Saturday. My trusty guitar cord must be getting loose so I switched to a new TRS cable and problem solved.
It's all about shielding. You mentioned hearing the radio station in your headphones. You ned to isolate what piece of equipment is picking up the signal - changing one piece of equipment at a time should help do that. Try a different set of headphones first (or use speakers). Then a different mic (same cable), etc.
It could be the voices in your head... just putting that out there.
No, it's most likely the shielding in your cables. I've had problems with it when i played electric guitar. I'm a fan of monster cables, i've never had anything go wrong with them... although they do usually end up lost or stolen.
So the MXL 990 makes a pretty good antenna? I may have to hook one of those up to my tuner... I agree it's probably a bad cable, presuming the problem came on alluvasudden like...
It could be other stuff though. Here's a link with a few tips, but keep in mind they are also trying to sell you a filter....