Do I Need to Replace One of My Drivers?

I have a pair of PSB 5Ts, and I recently noticed that one of the 5-1/4" midrange drivers exhibits a slight, momentary, barely audible vibration in a certain frequency range. I picked-up on this recently while listening to Jack Johnson. I noticed it (and had to confirm by getting my ear a few inches from the speaker) during certain classical guitar passages in the lower ranges. Vocal and drum parts don't seem to cause it. I have switched my speakers from left to right, so I know it's with one speaker and not my amp. Any advice? If I need a new driver, what is the most cost-effective way to get one?