As it turns out, I think the rattle was just the balancer shaft belt on the S2 motor hitting the water pump pulley. You can see how loose the balancer shaft belt was in a couple pictures 1,2 below. Probably only had to set the proper tension on the balancer belt and not replace the pump.
I did replace the cam chain tensioner while I was at it, and the nylon rubbing block was cracked along the leading edge fold.