sounds like it could be a wire rubbing against a cone or something. Sometimes a screw or staple from the insulation can come loose and get stuck in the frame of one of the woofers because of the magnet. When you really pump it it may get the cone moving enough to contact a foreign object in there. Just a guess.
Inspect the cones for creasing, and pull the drivers out, look for foreign materials inside the frames and spider area. Could also be a woofer screw.... Just do an overall inspection for anything loose.