Ok, I apologize if this isn't the right place for this. So, this chain of events has been going on for awhile and has 3 mechanics and a transmission shop stumped. Car - 52 styleline deluxe 4dr Engine - crate gm performance shop h.o.350 Trans - B&M th350 Differential - 56 chevy del ray Driveshaft - custom made to length through speedway motors. Car has been running and driving fine since initial build. Plenty of power for what i want. Transmission was bought used, so unknown history on it. One day driving down the highway i noticed what sounded like a "grinding" noise from the drivetrain when i would ease up on the gas and just try to maintain cruising speed of 55ish mph. Thought to myself, "great, time to rebuild the rear end" as i knew where it came from and it was next on list of getting a rebuild anyways. Put it in the shop, order complete new richmond 3.55 gear set up for it, all new seals, bearings, completely new from wheel hub to wheel hub. Go for a test ride, no joy. Noise still there. So then we all started wondering what the hell it could be. Thought, well, we dont know the full history of the trans, so might as well. I have a friend that runs a big transmission shop and has done me right in the past, so i trailered her up to him. Told him to go through it and upgrade the weak points and whatever he saw fit. A new tci 2200 stall was also installed at this time as well. Pick it up a few days later, he said the only things he found wrong internally was a couple of smoked clutches, but all line pressures etc read perfect. He said he put better sprag and couple other parts in it but it was in overall great shape inside, didnt notice anything broke etc.. Hop in it, drive it 50 miles home, once up to highway speeds....same thing. Gindish noise is still the at light load/ barely in throttle to just maintain speed. Get home, call him and tell him whats up and it is still being weird, so trailer it back up to him, he drops pan, checks things out for a couple days and says he cant find anything that points to transmission. Said if something was internally binding or wrong it would have never made it home. Car shifts fine, nothing seems off about the operation of the trans. So here we are. Still has a funky "grinding" noise at light load barely on the throttle. If you accelerate it goes away, if you let totally off the gas it goes away. Its only at one particular spot it does it and its unfortunately right where you cruise to maintain speed, so highly annoying. Have checked exhaust, seems to have plenty of room everywhere and not into anything, driveshaft is tight, everything is greased, crossmembers are tight. Nothing at all looks out of place. Soo here I am, asking the help of the all knowing h.a.m.b. Can anyone help a junior hot rodder out?