the bird is a 98 3.8L auto. there is a grinding like noise coming from the differential that i think is the pinion gear bearing. and i haven't been racing or doing burnouts and the most i've taken it up to is 60. however when the car is on and i shift to Drive or 3,2,1 there a noise that is coming right from underneath almost sounds like a thin metal flap that vibrating. then i shift into park and the noise goes away...any ideas?? thanks

So is there 2 seperate sounds? A grinding noise from the differential' AND a metallic flapping noise? Could you give us more info?

Yes of course 2 separate sounds. Watch it's like this. Again a 98 Black Bird 3.8L auto tranny with 112,xxx miles.

The differential noise can only be heard when the car is in motion. I've already taken it apart inspected for bent axle shafts, worn bearings, broken shims, gear wear, c-clips, wrong tire size, brake caliper getting stuck, etc. The only thing that is left has to be the pinion gear bearings. That noise was the first one that existed when my sista bought the car. We haven't been burning out or accelerating hard or anything. We've been meaning to take it to a professional to get that done but we'll use the car here and there. It also has the 2nd gear start.

That metal clunking noise is heard when the tranny is warm I guess. Today in the morning I started it up and shifted into Drive right away holding the brake to keep the car still and I could not hear it. After some time of driving it i'll come to a red light and you'll hear that sound almost like a metal flap vibrating like the one that protects the fuel tank or the cover to the flywheel, maybe even the cat converter. When I shift into park or neutral I can no longer hear it but the car runs goods and does not smell weird or anything. The tranny fluid level is just fine too. I just don't like hearing sounds like that, its unnatural.

nah turns out it was the catalytic converter, im surprised it went out just like that. but i still have to fix the grinding noise in the back. im sure its the pinion gear bearing.