I have an 8 3/4 742 rear, 3.23 Sure-Grip. I'm getting a high pitched noise on acceleration, either to maintain speed or increase speed. If I release the accelerator pedal, the noise goes away. The noise is worst at 45-55 mph and seems to decrease at highway speeds. The rear is in a 1970 Fury convertible. Tapered roller bearings on axle shafts are newly greased and endplay is correct. Rear does not make noise on turns. I suspect the noise is pinion gear or rear pinion bearing. Does anyone know how to distinguish between pinion gear noise and pinon bearing noise? I've heard that pinion gear is higher pitched, but could use some help on whether ring and pinion need replace. Thanks, Ben

ben how old are you u joints? ronnie

Not my area so a BTT for ya but more info, I was wondering if it was bad & you did the greasing/endplay to try & correct it? when did it start? any parts changes preceding this?

A pinion bearing bad may or may not make a rumbling noise, but a loose bearing will make the pinion pattern change and make noise.

I want to hear the answer too, I have a 741-3.55 sure-grip and does the same thing, same speed. Mine is a used set and thought it was bad run in before I got them. All ears.

Either the pinion bearing is loose/ worn allowing it to go forward under acceleration, or the differential side bearings are worn/ improperly set.
Either situation will allow the pattern to change and cause the noise you are experiencing.

The rear u-joint is new. The front u-joint is from before I bought the car 3 years ago. There's no play in either u-joint. Also, per my 1970 Factory Shop Manual, driveshaft length is correct.

[/quote] Not my area so a BTT for ya but more info, I was wondering if it was bad & you did the greasing/endplay to try & correct it? when did it start? any parts changes preceding this? [/quote]

The story is that I bought a 1970 Fury with bad rust for a parts car. I did drive it, but not at very high speed. I didn't notice problems with the 3.23 8.75 sure grip axle. My 1970 Fury convertible had an 8.25, 2.71 gear, open. I cleaned and painted the 8.75 rear from the parts car. I installed the rear in the convertible as prep for a big block conversion, which I have not yet begun. The axle shafts had a tapered roller bearing on one side and a non-adjustable bearing on the other, which was a mismatch. I had a tapered roller bearing installed on the side with the repair bearing, greased both sides, and set up the end play after I had installed the axle on the car. The noise appeared immediately with the replacement axle. I removed the pinion shaft nut and tightened to factory spec. I drove the car again and the noise remains.

MANY THANKS to TJP and Slantzilla. It sounds like the problem is the pinion bearings (or diff side bearings). Nobody is saying gears. I guess the good news is that I can keep my ring and pinion, but the bad news is that the center section has to be pulled and rebuilt. I'm only about an hour away from Fredericksburg. If you know a good rebuilder in our area, I'd be very interested in that info.

Not my area of knowledge but offhand that would lead me to believe that the problem was related to the endplay/thrust spacer areas rather than the gears cuz that was what was altered (tho you did say you did not run it up very high before the swap). Holler how it turns out