# Rear end noise



## kjk990 (Sep 1, 2010)

I had the rear end of my 67 GTO rebuilt last summer. I had changed from the 2.56 gears to the 3.55 gears. I also had an Eaton Posi unit installed. I also upgraded to four wheel power disc brakes. Once it was back in the car I found that the 3.55's were great off the line but not so fun on the freeway. I changed from the TH400 to a 700r4 a couple months ago. 

I'm in heaven. I love this combination. Now I am hearing the gears in the rear end rattle in the rear end when I'm at slow speeds. It's not really loud but I very noticeable. Mostly happens when I'm in reverse and more so when I'm turning in reverse. 

I pulled the plug on the pumpkin which has a magnet. I saw some small pieces of metal shavings. I am now putting it back together with fresh gear oil.

Is there anything else I should do? Is this normal for new gears to throw off a few shavings when they are braking in? Or is something major wrong? 

The guy who built it said it may make a little noise because of the size of the gears. But it never did until about a week ago. 

Thanks for the help.


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## BearGFR (Aug 25, 2008)

I would think that finding metal bits is never a good thing.

In all the information that came with my Moser 9" and Wavetrac, it did say that some 'noise' with a performance gear set is to be expected. In my case though it's more in the form of a very faint whine I can hear when cruising at a steady 60-70 mph or so.

Bear


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## geeteeohguy (Feb 2, 2008)

I agree with Bear. Some performance gears can have a slight whine from the way their teeth are designed and cut, but gears are very hard and don't 'make metal' that you can pick up with a magnet. Not unless they fail. Bearings and shims more likely to 'make metal'. The noise you describe sounds like bearing wear/failure. All bearings degrade from the date of installation to the day of failure, the key is to postpone the date of failure as long as possible. You said he changed the 2.56 to 3.55.....I assume he changed the entire carrier assembly. This is because the ring gear on a 2.56 rear end is super thin, and the pinion gear is huge. The 3.55 has a meaty ring gear and a small pinion gear. Some guys install what's called a 'ring gear spacer' behind the ring gear on a carrier assembly to convert it over to lower gears. This can work, but is generally not recommended for performance oriented cars. The statement that "it may make a little noise because of the size of the gears" could mean that he shimmed them (bad) or that he is telling you that a 3.55 is a noisy ratio because it's 'big'. The gear ratio will have no impact on how much noise a rear end makes. Just engine/drivetrain noise due to increased rpm.


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## kjk990 (Sep 1, 2010)

So I talked to the guy at the shop who built it. He told me that the amount of shavings, which is a small pile of metal sand about the size of your pinky nail, is not that uncommon for a re-build with all new internals. I haven't had time to take it over to him to hear yet, but he said the "chatter" noise is probably from the limited slip unit. He told me to add a gear oil additive called "friction modifier". I added it, drove around a little, then tried to make the noise again. It was gone. 

I'll keep an eye on it. Hopefully he's right. I don't feel like having it re-built again or changing it out for something stronger.

Thanks for the help.


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## geeteeohguy (Feb 2, 2008)

I didn't realize that you didn't add the positraction/limited slip additive initially. It's critical, as you found out. No damage done, no worries. It should be fine for a long time. This is a common issue and it pops up all the time.


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## kjk990 (Sep 1, 2010)

It seems to have done the trick. I'm glad thats all it was. Wow, aren't $5.00 fixes great?

Thanks guys.


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## chuckha62 (Apr 5, 2010)

I'm surprised your rear end guy didn't know about nor add the friction modifier from the start. GM knew about it and added it from the factory when these cars were built. 

When I rebuilt the differential on my old Nova, I went with the Factory style Eaton limited slip carrier and then I spent an hour on the phone with the Owner/Chemist of Redline Oi, because I wanted to get it right. He assured me that his synthetic products were formulated such that the friction modifier wasn't needed. I went away fat, dumb and happy. That is, until my rear end started to chatter. I flushed and replaced the Redline with GM Posi-traction Gear Oil and their friction modifier. It all quieted down and all seemed well. 

Two years after I sold the car, I found out that the new owner had to replace the Posi carrier, as the clutches were worn out and most likely this was due to the wrong product being used during break in. 

Now, I'm not bashing Redline's products, but they are not the be all, end all. Whenever I'm in doubt, I find that I generally default to what GM knew and did from the factory. 99% of the time you'll do best by following their recommendations. After all, they had to warranty their products and that can cost $$$$.


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