Rob, what, if not the drives, was the source of the rattle?

It was the bottom of the RioCar rattling against the bottom of the sled on one side. Perhaps my sled is installed slightly crooked (looks straight to me) but it only rattled on the left side. My sled is for my Mk2, so maybe there is a slight difference in sled designs or the Mk2a is slightly thinner than Mk2. I was going to try an fix it by putting that tape that feels sort-of like fabric over the bumps in the sled.