Nobody has asked you about your calibration settings. It is possible your running the sub to hot. Have you calibrated everything to 75dB? Do you have an SPL meter from Radio Shack?
If you have the volume on the sub cranked to high, AND have the receiver sub dB levels to high, the woofer will really work hard.
Or, it could just be the port rattling, or something on the back amp panel. I had a rattle once in a 500 I owned, and if I applied preassure with my fingers on the back of the amp plate, the noise went away.
