James, To me that sounds like either a blown speaker(or ready to go) or it could be a loose connection in the door itself or at the HU, and it could also be a bad ground at the HU...If its a factory system i would bet its blowing out, but aftermarket could also be bad connections...If you had it installed somewhere, have them take a look at it...Most credible places have a lifetime warranty on their installs.
Good luck
Andy