I have a 1970 chevelle with a turbo 350 and a medium built small block. I just had the tranny overhauled and upgraded with performance parts by a reputable shop. I have put 50 miles on since with trouble free driving for the first 30 of them. During the last 20 miles of driving there has been a grinding noise comming from the area at the front of the transmission tunnel. I had some exhaust work done which caused my headers to hit the blower motor cover of the car (i have a air delete cover on because i took of the air) the headers did not hit the cover before the exhaust work was done, nor did this metal grinding noise happen. The noise sounds like a bad wheel bearing it only happens in neutral, park, and reverse but not in forward gears. When accelerating and holding the car in drive at a stand still the noise abbruptly stops, however the noise is still apperent while holding the car in in reverse. It also is present when holding the car or parking the while car facing downward on a hill, however while facing upward it disappears. It also happens if the car is prematurely put into park but only makes the noise when the car rocks forward, and not when it rocks backwards. The noise increases with engine rpm when it happens. I have never heard or anything like this happening before. The converter bolts and flex plate bolts are tight and the flex plate is not cracked, there is plenty of fluid as well, the tail shaft is not rubbing against the body, and nothing is caught/pinched on/in the case of the transmission. What could cause this noise to happen? could it be the front transmission pump or could the header be rubbing on the blower box be causing this noise???

Sounds like your fan blade maybe hitting shroud. Check your motor mount on driver side.