Without getting too technical, you have a short. The back covers are shielded for a reason, and a portion of the metal may be touching the board itself. "Duh, I know" is probably your response, but take a peek also at the edges of the casing... are their any soldered connections that are loose? The unwanted connection may not be to the board, but perhaps the USB port or headphone jack. Crazy? Maybe, but I've seen sillier "bad connections" that have caused more trouble!

Hope this helps!