My 99 Forester with 167,000 miles is making an odd knocking sound from somewhere in the engine. I only hear it at idol with my head under the hood. Could this be the waterpump bearing failing? It was replaced with the timing belt at 90,000 miles. This sound a lot like the sound my 1980 dodge cold water pump made when it was failing.

Timing belt tensioners on those engines will knock around when they get old. Did you replace the tensioner when you did the timing belt?

I don't know.

I take it that means someone else (shop) did the work?

 

Dig through your service records and see if you can find out which, if any, other parts were replaced along with the belt and water pump.

 

Another common issue on these, but generally shows up much sooner, is the bolt on the crankshaft pulley is not tightened properly after a timing belt change. This allows the bolt to loosen up and the crankshaft pulley to start wobbling around making a knocking noise similar to a bad rod bearing. If this is the problem you will be able to watch the pulley wobble as the engine runs.