After time, all speakers and subwoofers become old and blow out. However, if the audio system in your car is popping or rattling, the problem is not necessarily that your subwoofer has blown. There are a few key ways to tell if that really is the problem.
Leave the subwoofer connected to your vehicle's audio system, but pull it out of the vehicle.
Lay the subwoofer set on the ground, and turn on the car's system. Play an audio track with a good amount of bass.
Listen for popping coming from the subwoofer itself. If the sounds coming from the subwoofer do not match with any oddities that you heard in the vehicle, the problem may be a loose part or accessory in the vehicle rattling around.
Use a flathead screwdriver to gently pull back the lining keeping the subwoofer head within the assembly. Check the entire surface of the subwoofer head for cracks, dents or other forms of damage.
If the subwoofer is not cracked and does not make any unexpected sounds when outside of the vehicle, check inside the vehicle for loose parts or other things that may cause popping or other sounds that would seem to indicate the blowing of the speaker. A common cause of these sounds is a loose boot lock.
If problems continue to occur, have a professional audio tech inspect your vehicle for you.
Always use caution when working around live electrical ends.