[roddy]

I had the rear fender liner rattle, and it sounded a little different than what you describe. That could be it though. If you want to find out, the fix is pretty easy. Go to your local hardware store or Home Depot, and grab a handful of neoprene (rubber) washers in 5/16" and 3/8" sizes. Pop out the clips (push pins) out one at a time, and slide a washer under the head of the pin, and pop it back in place. It's been a while since I did this, but I think there are around eight 5/16 and three 3/8 washers needed per side. If your fender liner is moving around, this will stop it, and the noise.

Edit: I just re-read your post, and like yours, bumps and such didn't affect mine either. It was highly irregular, but most noticeable on a windy day (cross winds), or when passing a large truck on the highway. Keep an eye on when this happens, if the circumstances are similar, I'd bet it is the fender liner causing the noise. In my case, it was the passenger side, and sounded like it was actually coming from the back of the rocker panel area.