You may need to pull the driver and play it while it's out of the box to get a better idea on whats going on.
Try tightening all the screws and bolts you can get to from the outside first. Check the port to be sure there isn't something in there.
Unless you see something from the outside it is going to need to be taken apart. Something has come loose inside (amp? bracing?) or the speaker is coming apart or rubbing.
I've been tearing apart / repairing a lot of drivers for other folks lately. Most of these drivers were thought to be on their last leg and beyond reasonable repair. After careful inspection, i've found that many manufacturers are not using strong enough adhesives to secure the dustcaps to the cones and / or the spider to the voice coil former. Either of these can produce the sounds that you describe. If not taken care of when first noticed, either of these will lead to an early demise of the driver itself and poorer performance on the way to the grave.
Both of these pieces are subjected to high levels of physical force AND thermal stress. My guess is that the adhesives being used aren't up to the task at hand and / or they simply aren't using enough adhesive to begin with. By simply applying fresh adhesive in reasonable quantity, all of these drivers have not only survived, but improved sonically.
As is usual, it is much cheaper to repair a problem than have to replace the entire unit, so take care of your specific situation before it gets worse. Sean
My thanks to all of you who have responded. Sean, I appreciate the in-depth explanation regarding the adhesives. Certainly makes sense. I removed the driver from the box and operated the system. I could still hear the pop-pop sound with each beat of the song being played. As I turned down the volume from my amp or the volume down on the back of the sub, the pop-pop disappears. It seems to only do it when it's being driven but not driven beyond its capabilities. I have retightened all screws but to no avail. The pop-pop is coming directly from the driver itself. If it is an adhesive problem, I'm just going to return it under warranty (two years on this unit from B&W). Thanks again.