Check to see if the screws holding the speakers in are tight. If that's not it try swapping speakers from side to side to see if the problem switches sides.
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The first thing I would do is exactly what Sarcher30 recommends. Drivers come loose all the time. When it comes to swapping the speakers, though, I would do it both ways. First, just swap the speaker cables. If the problem now moves to the other speaker, chances are the problem is somewhere else. If the problem is still there, then move the speakers. The reason for doing it this way is that you may have an issue with the room that may fool you into thinking its the speakers. Switching the channel and then the location gives you 2 separate verifications as to whether you have a speaker problem, or not.
After doing all of this and you still have a problem, can you better describe the sound you are hearing? More details will help you narrow the problem down to a solution.