A while back before the snow fell, I took my 69 out. Ran great, but I got this strange noise from my tranny in 3rd gear. Its kind of like the noise you would hear from a vibration, but I can't feel any vibrations through the shifter. It doesn't happen offer and it only happens in 3rd gear with part throttle and when I let off from the part throttle. If I hit the gas a little harder it goes away. I had my ST-10 completely rebuilt with all new gears, input shaft, bearings, brass etc. Everything inside the tranny is new. I've only put on around 300 miles since the rebuild. I'm wondering if my shifter is out of adjustment or if this is part of the break in?

I would run the car to get the trans oil hot, then drain it and throw a magnet to see if steel was attached.

If there is steel powder or bits of steel on the magnet, save it for the trans builder to evaluate.

Muncie Rock Crushers had gear noise due to the tooth angle used. ST-10's I don't know. But if its only third gear, I would suspect either the needle bearings in the rear of the input shaft going bad or the input shaft itself. Because these needle bearings hold the front of the output shaft.

By placing the trans in 3rd gear, this places a greater load onto these needle bearings and input shaft as opposed to 1st & 2nd gear.

If using synthetic gear lube, this oil will allow louder gear noise than petro base oil. I believe petro 85W140 GL5 is the oil to use, a minumum of 80W90.

I could be wrong here......

Thanks Everett. I know this is not the regular gear noise. I hope that neither the needle bearings or input shaft are bad since they are all new. I tried synthetic oil, but it leaked a little so I went back to regular Valvoline 80w90. The oil has been changed twice since its been rebuilt. The oil was a little gray, but I figured this was from everything breaking in. I didn't notice anything out of the ordinary. I had it rebuild over a year ago and I've moved away from the shop that rebuilt it. I don't plan on putting any more $$ in this tranny or pulling it out again. I just wondered if the shifter adjustment could have been off. If not, I'll just live with the noise every once in a while.