Something sounds loose. Could be a wire too close to the driver vibrating? Loose screw perhaps? Or driver suspension flapping?
I would remove the amp again. Pick up the enclosure and shake it to see if you hear anything. Also shake the amp to make sure that isn't the source. Look inside with a flashlight to see if the wires are too close to the driver. Also look at the driver suspension closely to make sure there is no minute damage anywhere. Tighten everything that you can.
Still makes a noise? Hang garlic and crosses in the room to ward off spirits and consult Randy.
