Turns out the problem was in the center of the pad. My drums have a black plastic post held on by two screws on the bottom of each drum pad. The screws had loosened a little, allowing the posts to rattle a bit each time I hit red or yellow. I tightened them until the posts were locked in place like the quiet pads and haven't had any more issues. I'm not sure if this will last, but if the rattle comes back I'm going to super glue the post in place, backed up with the screws.
I have an early version of the drums according to the bottom of this page (no extra tab on the bass pedal) http://www.woodrockpedals.com/ and all four pads pop out with a little leverage - no screws to undo first. I thought this might save somebody some troubleshooting time.









