If it wasn't coming from only one woofer, I would suspect that the problem is elsewhere. If it is coming from one woofer, it could be that the voice coil is somehow rubbing or there is some debris caught between the voice coil and the magnet. If either is the case, you may have to replace that woofer. You could test this if you felt adventurous. If you swapped woofer from the right and left speakers and the sound moved to the right speaker in the process, you would confirm that it is a problem with the woofer.
- Jim