Sounds like a propeller shaft U-jioint, that's what mine sounded like. R&Ring those shafts is not a lot of fun. If you're going there though, make sure you're wheel bearing is in good shape at the same time, cause you gotta go through the whole proceedure again if the bearing is bad.
Also, if your driving the car real hard or putting any extra power to it you might want to ask around about the kind of u-joints to put on. I put aftermarket ones with grease zirks on, and it seems like a lot of people don't think that's a good idea.
Get the car up off the ground and spin the wheels, you should be able to figure out what it is. Also, if ones going you might want to replace them all. I couldn't detect any trouble with the others during inspections, but my wheel bearing went bad on the side I hadn't replaced and when I pulled that side apart they were well on there way to needing replacement. Good Luck.
