That was with the engine off, eh? Interesting.
I'm assuming it does it with any audio source, FM, AM, CD, etc..?
Follow these steps to make a silent CD: http://legacygt.com/forums/showpost....&postcount=381
and play it back so we can get a better sampling of the sound.
It's all just experimental at this point. Short of having the head unit replaced (still under warranty?) I don't know what would fix it.