My 2003 Monte SS 3.8 has a rattle. It sounds like it's exterior and not inside the engine. I have already changed the pulley tensioner, changed the starter, AC compressor was replaced last fall, belt is new and alternator new.It sounds like a heat shield or something loose, but I can't find anything at all .... including the flywheel cover.I was sitting in the car today, totally frustrated, and I turned on the heater. The rattle goes away when the heater is ON.
Does this possibility mean the water pump needs to be changed? Looking for suggestions, PLEASE!
ANY IDEAS WILL BE APPRECIATED!

 

Its hard to diagnose something like this without being there in person. If you do think its the water pump (or anything else on the belt circuit) making noise, then try just taking the belt off and starting the car. Obviously don't leave it running for long - but if the noise disappears then you've isolated the problem to that area and need to start inspecting each component there closer.

 

Turning on the blower may raise the idle enough to smooth out the vibration. ..Resonant vibration frequency mumbo-jumbo.

I once had the same situation that turned out to be the crossover heat shield.

 

I once had a rattle that I tracked down to a piece of spare pipe in the glove box.
I felt stupid but relieved.