SM465 Noise 4th gear on power

[palidine556]

I have a 1985 Chevy C30 with a 454 and SM465. Recently on a long trip i noticed a noise coming from somewhere underneath the truck. I jacked up the rear end and ran the truck in 4th gear to try to isolate where the noise is coming from but since it wasn't under load it was quiet. I have taken off the differential cover for the rear end and the PTO cover on the transmission and did not find and chewed up gears or metal shavings. The sound is very obvious in 4th gear. It is only when i am giving it gas, if i coast everything sounds fine. If anyone has any clues to help figure this out it would be greatly appreciated.

 

[smurph20]

Check your ujoints

 

[palidine556]

Checked all the U joints and they were all tight.

 

[HotRodPC]

I'm going to go with bearings and not input shaft bearing. If it were input shaft bearing, it would be noisy in every gear, but shut up and be quiet in 4th gear.

As you're putting torque on the gears they're feeling the pressure aka thrust. When you let off to coast the pressure is off. How about gear oil level? These transmissions can be noisy forever and keep on going and going and it's usually bearings.