I have a 1991 pontiac firebird 305 tbi.
At low ide or 1st gear.. I'm getting this rattle that is intermittent, I have changed my water pump, compressor, idle pully. It not the exhaust. any suggestions.. I'm thinking either the water pump is bad.. or the alternator.. because when I rev up.. the sound goes away..

 

Lots of stuff this could be. Remove fan belts temporarily, run the engine, see if the rattle disappears. If it does, it's in the accessories--AC, PS, alternator, water pump, AIR pump, etc. If it doesn't, check the exhaust again, verify that the motor mounts and trans mount haven't collapsed or degraded, and that the torque converter bolts are tight. Assure that the flexplate isn't cracked.

Beyond that, verify all heat shields--exhaust manifold, catalytic converter, and any others on your vehicle.

Take all the junk out of the trunk, clean out the back seat, and make sure the tire-changing jack is properly secured.

 

I have a ‘92 Firebird 305 V8 TBI and it rattled something fierce at idle, drove me bonkers. You could really hear it when pulled up at a drive through. It ended up being that the shaft for the thermostatic air valve in the air cleaner snorkel was missing and the valve/flapper thingy was flopping around inside the snorkel. I used a piece of brazing rod which was the perfect diameter, threaded it through both the snorkel and the valve and then cut it to length using a Dremel and bent the ends over. No more rattle and performance has improved too. Crazy, eh?

 

 

Had something similar with mine and it was the AIR pump had worn out. If you have a car stethoscope from Harbor Freight for like $6, it can help narrow down where the noise comes from. Good luck with finding it.