I had the noise you explained on 2 occasions with my EP500. With the first, a setting in my Denon amp had it running 10 DB hotter than what it showed for DTS DVDs (or was it for non-DTS, I can't remember now.)

Second time, I actually found a cats toy stuck way back inside the port. This was both a mystery and an embarrassment to me as the cover is always on and I had also been speaking to Axiom as to what the problem might be.

With great power comes Awesome irresponsibility.