Need Help, Center Speaker Distorting!!!!

[snsguy]

Hey Guys, I need some help. I've been hearing more and more distortion coming from my Polk CS2. I've tried a bunch of different stuff but for the life of me I can't figure out what is causing it. It's happening a lot with Dialog. Certain scenes I am hearing a buzzing or kind of a crackling noise coming from the center speaker. It was slight but has gotten worse over time. I'm not sure if it's the receiver, which I just got back from repairs about a month ago, or the speaker itself. I've tried switching my two Monitor 60's, left and right speakers, to the center channel and will hear the same distortion but it's more tame. I've tried my back onkyo surround speakers and may or may not hear it coming from them at all. They are not the best so I'm not sure if I would here from them or not. It's really aggrivating me though because I'm hearing it a lot. I heard it a few times in Prince of Persia, I hear it a lot when watching cable and in a few other movies that we've recently watched. It's always most prominent with dialogue and it almost seems like a certain frequency. I've tried changing out the speaker cable and that didn't do anything. Any help would be appreciated.

SNSGUY