Sounds like the voice coil(s).
First make sure that the braided wiring going from the sub cone to the voice coil(s) have not come loose. Examine the braided wire all the way from the terminal to the cone. If one or more has come loose, it can sometimes be resoldered. Just be careful with the heat.
If all the wiring is intact, you can check the coils with a multimeter. With the sub disconnected, and the meter set to the lowest resistance scale, test across the speaker terminals. Depending on how the coils are wired you should measeure somewhere between 1 and 4 ohms. If you get "0" or no reading at all, one of the coils is open. If you get "infinity", a coil is shorted. Sometimes only one coil is bad and you can still use the sub, wiring it to the remaining coil only. Once you have determined that one set of coils is either open or short, you can remove the jumper and test each coil individually.
You can test most any speaker voice coil with a small battery. The best to use is the 9V type that has terminals right together. Connect a short piece of speaker wire to the sub terminals, and hold the other end of the "+" wire to the positive battery terminal, then quickly touch and remove the negative battery terminal with the "-" wire. The sub cone should move outward, and you should hear a brief thump. If no cone movement, and no thump, the coil is defective. Do not hold DC to a speaker for any period of time though, just touch and remove. And NEVER use a high-current battery, like a car battery for such a test. If the coil is not already bad, high DC current will damage it.
Either short or open, replacing a voice coil is a job for the Rockford-Fosgate professionals.
on Apr 18, 2009