I got a sub from my neighbor and it rattled when playing but I figured the woofer was loose maybe. But when I opened it up a screwdriver was stuck to the magnet and what was vibrating.
Back in the day my wife and I were in the Bay area and visited "Zilch Labs." Zilch was demonstrating one of his creations and I told him I heard a slight "buzz" or "rattle" in one. He never heard it and passed it off as me "hearing things."
The next year I got an e-mail from him. It seems that when he opened the cabinet, he found a loose screw that that stuck on the woofer magnet and was rattling along with the woofer. He never doubted my hearing after that.