Thumping noise

Hello,

I have been having a strange noise coming from my sub for quite awhile now, it is hard to describe but it sounds like very low fast thumping noise. I have brought my sub to our local audio dealer and has it sent in to Paradigm and they could not find anything wrong with the sub or get to noise to show up. The people that install my entertainment system can not figure out what is causing this, We have tried everything, a different outlet, new cables (not cheap ones). One thing that has made a difference is pulling the sub out of the cabinet. My sub is located on a movable shelf that I can pull out of the cabinet for easier access. But as soon as I push it back in the cabinet the noise returns. We determined there is some kind of interference  but can not pin point where it is coming from.

I can try and take some pictures of the cabinet and try and get an audio recording of the noise if that would help. Another strange phenomenon is it will make the noise for awhile then just quit but it eventually returns.

Thank you