Rear End Noise

[hevy_chevy67]

Yesterday I swapped out my entire rear from the narrower 67-70 style to the 71-72 style. I bought the replacement one from a guy parting out a project and it had been gone through. 3.73 posi, everything looked good to me, but I'm not expert haha.

Anyways, it's making a whinning noise once I get some speed going. Below 30 mph everything sounds fine, and even on acceleration and when I let off the gas the sound is completely gone or at least nothing that I can hear. It seems to rear up when just feather the gas or when under light load. Any ideas? I'm going to go get it checked out but it's pissing me off. Driveshaft was recently balanced and no such noise existed with the old set up in there.

[HO455]

What ratio? 12 bolts have a reputation for being whiny in the higher numerical ratios. Basically 4:10 and above.

[hevy_chevy67]

Originally Posted by HO455

What ratio? 12 bolts have a reputation for being whiny in the higher numerical ratios. Basically 4:10 and above.

It's a 3.73.

[Keith Seymore]

When an axle does generate noise it is typical for it to whine on accel - or - coast - or - decel. But rarely more than one of those conditions. So your experience is typical in that regard.

It also occurs in a very specific range because the driveline forcing function is not that powerful compared to the mass of the vehicle, meaning it needs the multiplying effect of resonance in order to generate enough power to be a disturbance. Not like a tire problem, where usually once it starts it just keeps going as you increase speed.

It could be a result of an incorrect setup in terms of pinion depth or backlash. If it has been "broken in" in this configuration then it has likely taken a set and there may not be an option other than changing the ring and pinion to a new setup.

Driveshaft balance has nothing to do with it. Imbalance is a first order of rotation disturbance; gear whine is a function of the number of teeth on the pinion gear (41 divided by 11 = 3.73, or 11th order of rotation for you). Every time the pinion tooth hits the ring gear it creates a "kick" causing the drive shaft to resonate torsionally, and that's what you are hearing.

K

[HO455]

Originally Posted by hevy_chevy67

It's a 3.73.

I would suspect the ring and pinion mesh. It may not be fixable without a gear set change. But I am no expert. If Randy500 doesn't respond send him a PM. His knowledge about rear ends far exceeds mine.

[gmc684x4]

Did you put in posi additive?

[hevy_chevy67]

Originally Posted by gmc684x4

Did you put in posi additive?

Yeah I put in a bottle of the AC delco stuff and 80/90 oil. No chirping or any sounds when I'm turning.

[Ironangel]

Check the backlash, and check for end play on the pinion shaft. The GM 12T should have .006" to .010" of backlash and zero end play...I'm thinking you have a rear end that was never correctly set up, and/or a pinion shaft that has end play due to overtightening the initial preload on the crush sleeve. All repairable if you pinpoint and address the problem now. Jack the rear axle up until both wheels are clear and set it on stands. Pop the cover and capture the lube in a clean pan for inspection and reuse if clean. Pull both axles just far enough to clear the splines in the carrier and attach a mag base dial indicator to check the backlash on the ring gear. You can get under the truck and push in and pull out on the pinion yoke using the drive shaft. There should be zero end play. If there is end play plan on buying a $4 crush sleeve and a $3 pinion seal and re-set the pinion preload. I took the whine out of one of my 72's by simply tightening up the loose backlash by swapping shims from the left side to the right...Here's a manual, https://www.randysworldwide.com/wp-c...l_Complete.pdf

[hevy_chevy67]

Thanks for the comments everyone.