Rear Differential Groan Noise

[Bill B.]

Hi everyone!

I have an issue with my rebuilt 1970 posi-traction differential unit with now over 3,500 mi on it. (I did not perform the rebuild myself) When I take corners at low speed, I am getting an audible "gronk, gronk, gronk" noise with a physical vibration I can feel from the cockpit.

Some history: After rebuild and installation two years ago, I had originally used 80W-90 GL-5 dino oil with two bottles of the GM additive. Mild mechanical fetching when I took sharp corners. Not unusual and thought it would get better with time. After about 600 miles when I changed out my differential fluid, I figured I'd try the available GM synthetic differential fluid with two bottles of additive. Didn't notice any difference at first, but over time, (past 18 months) the slight mechanical fetching when turning, morphed into a mechanical groan, which seems to be getting louder and louder over time.

I am soliciting advice if I should go back and re-service the differential fluid with the dino GL-5 lubricant? I hate to drop the differential for mechanical rework if I can avoid it. Hoping to hear from others who may have encountered similar symptoms and what their resolution was for a cure.

The noise is so prevalent, that I am embarrassed to perform sharp turns or back out of parking spaces without people turning their heads my way and wondering if my Corvette was going to come apart right there ...

Thanks!

[Patrick H.]

Re: Rear Differential Groan Noise

Have you checked everything else?
U-joints? other areas?
There are several items that can make that noise, not just the differential.

[Gary R.]

Re: Rear Differential Groan Noise

Bill
Eaton has always warned against using synthetic oil in the diff. They tag new loaded posi's not to use synthetic.

It does sound like posi chatter. When the posi was rebuilt do you know what type of clutches were used and how they were setup? for instance when I build a posi I run them in on my machine which is equal to about 800 parking lot figure 8's. I am not suggesting you do that, just wanted to know if you knew how the posi was setup. Could make a difference, such as with fiber clutches they smudge away so much the oil is black.

The easiest thing to do is change out the oil again, using old school gear oil and additive. I always run 85-140 in my diff's and I like the Lucas oil. I stock the GM additive and use 2 bottles of that.