I spoke with the gents at Furman sound about this problem round about a month ago, and they had a pretty good idea. The guy asked me, "well, what station was it?" and I said "I have no f'ing clue," and he said, well, figure out what station it is.
THEN you take an actual radio, and tune that baby to that station. Then you wander around the room seeing what kind of signal that baby picks up. The furman fellas said that oftentimes you set your shit up in one place, and you start hearing the junk, and so you change all your cables and disconnect your whole board trying to find it, when, as it has been said, you're walking around with in a giant antenna. Sometimes moving everything just a couple of feet can kill it.
Of course, they also listed cables as the main culprit. My main offender was a 9 dollar p.o.s. guitar cable, and so I upgraded and that was that.
The channel I was picking up was... no joke.... bible talk. It sounded really cool when you cranked it up and and added distortion.