Coolant Pulley play...
W202 coolant pulley play - YouTube

I took the belt off and checked the tensioner. It spins with a ticking noise... not sure if that's normal. I also checked the coolant pulley for play, and as you can see in the above video, there is some play that causes a knocking sound identical to the video with the engine running. Replacing water pump should do it? Or should I replace the tensioner while I'm at it as well?

Tensioner video:
Nov 19, 2013 - YouTube

Thanks guys!