91 grand am engine noise!!!!

i recently bought a 91 grand am and it had a timing chain cover leak....
i was getting around to fixing it when one day i was coming home from work and the oil light came on and the engine shut down so i pulled over and found 4 quarts of oil in my trunk.. put them in and got about 2 miles and it was out already .... i still had about 2 miles to go to get home so i PUSHED IT .... babied it home but the engine started making a loud "RAPPING" noise ..... so n e wayz i fixed the timing chain cover leak and took off the oil pan and low and behold the rod bearings where like paper thin..... so i replaced them hoping thats what was giving me that rapping noise ..........BUT it wasnt , i checked all the rods -straight-
checked the rockers -tight- ,took the head off- valves seated correctly,
i cant figure out what the heck this noise is SOMEONE HELP!!!!
im thinkin tis the output bearing beacause it dont do it when u have load on the car (going uphill and things like that) u hear it when its in park and u rev it a lil bit and the load is about to come down but u push the pedal a lil bit harder and it goes away ,as long as u keep load on it u dont hear it ...........
AM I RIGHT ABOUT THE OUTPUT BEARING????????
need input immediately

it sounds like the main bearings are gone . THey make a deep thumping sound that you can here when the throtles held at a constant rpm(3k or so). which sucks since you just had the oil pan off. Get some plastic gauge and mic the main bearings, I'd bet thats the culprit.