Hi,
So are you saying if you just plug the sub power in and have all other wires disconnected (ie RCA) the sub makes this rumble noise?
If this noise is made with the rca plug , plugged in then I would say to try a better cable. I had a similar situation , where I had grabbed a set of rca cables that had been in my sons car. They worked fine for a few months then one day I noticed some odd rumblings, long story short I changed to better and shorter cables and problem solved. Sub amps are sensitive and any inductance in the wire will be reproduced.
