Why is my subwoofer randomly hitting?

Why is my subwoofer randomly hitting?

Recently my car's subwoofer system would cut out and make a random fast thumping noise. It's quite loud and I immediately would turn off my head unit fearing it would blow my sub. I replaced the RCA cords, and checked all of the wiring and it all seems OK. I've searched online and the only similar problem is the thumping is similar to a heartbeat, which isn't the case.