1) the volume at which you were using the speaker in at least one of those videos looks like it is pushing the driver to its maximum excursion which can damage the driver. This will create a solid thumping noise as the driver cannot pull backward any further into the voice coil.
2) the noise in the back sounds like either a loose cable inside, a loose part on the circuit board or a loose screw which also would sound louder if the speaker is pushed harder
I suggest you contact Axiom customer support line via phone for the fastest response to your concerns.
(Incidentally, if you use the EP400 at such volume levels, you might want to consider a larger subwoofer that has greater power and output capability.)
Edited by chesseroo (06/04/14 11:49 AM)
"Those who preach the myths of audio are ignorant of truth."