Changed out the rear differential fluid today and bingo the noise is gone. It developed the dreaded posi-clutch chatter around 6500 miles on my 06. If you haven’t experienced the chatter it’s extremely annoying. When backing out of a garage it sounded like the rear end was falling apart. I read all the threads and decided to change out the fluid myself. I really don’t like the dealer touching my car unless absolutely necessary. I decided to use Amsoil SEVERE GEAR 75W-90 Synthetic EP Lubricant and Amsoil Slip Lock Differential Additive. After reading all the treads I went with 2 4oz bottles of the additive. I also purchased Amsoil gear lube pump (11 bucks) and it worked great refilling the differential. You simple screw the pump right to the top of gear lube bottle and you're ready to go. I also premixed the gear lube with the additive and then put it back into the original container gear oil container. If you don’t I think getting the additive in the differential is going to be difficult. The additive mixed very well and seemed to be well disbursed with the gear oil.
The service manual called for 54 oz of fluid (1.69 quarts) but when I drained mine it had closer to 62 ozs of fluid. A fair amount of difference I thought. Therefore I decided to replace mine with close to the same amount drained. The entire process took about an hour start to finish! I then took it out for a 15 mile drive. Stopped and did the old figure eight which I’m not sure does any good but what the hay. I also did several good burnouts to check and see if the posi clutches were working properly. No PEG LEG burnouts so I’m satisfied that 2 bottles of additive didn’t make the clutches to slippery. Bottom line is this is a very easy DIY if you can get the car in the air and have the proper tools. Now the question will be how long the clutch chatter stays away. For those that develop the posi chatter this is an easy and worthwhile fix... Happy motoring..

 

Thanks a bunch for the post and the details, i picked up my 08 and if i start to hearing anything i will change the fluid out too, is it bad to replace it with the mobile 1 ? I also thought that the additive was already mixed with the gear fluid... Let me know...

Thanks

 

Thanks for the info (and encouragement). I'm debating whether to let the dealership do it . . . not concerned they'll mess up, just more convenient to do it myself. Perhaps I'll follow your lead.

FM

 

No.. the slip modifier/additive is sold separate and made specifically for posi rear ends. I went with Amsoil because several posters said they had really good luck with it. It was also very easy to obtain because I ordered everything I needed from the Amsoil website. I went and bought a HUGE drip pan figuring I'd have some fluid mess... not needed IMO. I had a decent size oil drain container (autozone) and that was all I really needed. Maybe a couple rags and some cardboard to be safe. BE SURE to remove the FILL PLUG FIRST... makes draining much easier. Check the "Sticky Section" because there are some great write ups with pictures. I printed them out and used them as my guide. Very detailed instructions so you can't go wrong. Just make sure you have a 10mm allen socket to remove the differential plugs. It's a very easy DIY and worth doing if you ever get that posi chatter.

 

Amsoil is a good choice. It is best NOT to over-fill the diff; I understand that many are shipped this way.

 

Good post. I just hit 7k miles in my 06 vert and the clacking has begun for me also.
I recall reading a couple of posts that referenced a GM service bulletin about the clunking/clacking. The rec was to change out the fluid with a different spec fluid.
Will the dealer change out the rear axle fluid if you take it in with that complaint?

BTW, I have an excellent dealer service dept. in Greensboro.

 

There are several TSB's on this problem so this should be under warranty. I live about 35 miles from my dealer and it's easier for me to do the work when possible. I pretty sure any dealer could handle this one without a problem.......

 

My 06 A6 differential had the fluid replaced at the dealer 3 times and now with 15000 miles it is making noise again. I will be calling the dealer tomorrow but don't know what the long term fix will be.

 

the chatter lets you know you bought a performance car!! posi is one of the aspects of a awesome car. anyway its fun when you take someone for a ride and tell them the rearend might break so hold on!!!!!

 

I strongly suspect there is no long term fix! GM's had this problem ever since I can remember with posi differentials. Some folks claim to have had better luck with products other than GM fluids. Don't really know if that's accurate however. I find it interesting because not everyone experiences this problem? So why would that be if the all the rears on reg C6 are the same? I know some have had the posi unit changed and a few months later same noise! I'd say just keep changing out the fluid when the noise gets to annoying. After the first change it won't take more then 30 mins to do it.. not a big deal really. Think of it as changing your oil....

 

I need to do it on my 06 a6, guess i will have my Vette shop do it.

Not letting a dealer touch it!

 

You should really try replacing the oil with Amsoil. Many of us (including myself) have had very good success with it. Many have let the dealer change the oil, only to have the chatter come back. I am not aware of anyone having their chatter come back who has replaced their oil with Amsoil and the Amsoil additive (but I could be wrong). Give it a try, you'll like it. Very easy to do.

 

Given that there doesn't seem to be a long-term fix...

Will any damage be done if you don't do anything and just live with the noise?

 

Since Amsoil isn't GM approved wouldn't it be hysterically funny if indeed it was the long term fix! I'd be happy if I could get 10,000 miles before having to change the fluid again.

 

As there is a drain hole and a fill hole and the service manual calls for filling until fluid flows out of the fill hole, wouldn't it be hard to accidentally overfil?

I'm surprised that the Amsoil doesn't meet the GM spec?

 

I've read here where GM claims it's not something mechanical to worry about but simply a noise or annoyance issue. I find it hard to believe that much clanking isn't eventually going to cause mechanical problems but I don't know. Honesty mine was reaching the point where it was embarrassing like sort of the fart in church syndrome
It really annoyed me a 50 grand car had to sound like that backing out of garage every time I drove it. I hope the Amsoil solution works as well as lots of people have claimed.
One thing is true it's so easy to change even a CAVE MAN can do it....

 

Yes... and when I drained mine it was very close to 2 qts of fluid not 1.69 qts. The original factory level was right at the fill hole because I made sure to check it. I simply put back in the same amount that came out.

 

Can you give some reasons as to why you say this?

The reason I ask is because I've owned at least 30 GM cars with posi and NONE of them ever made the noise my 06 Corvette posi makes.

Of those 30+ cars, 14 of them have been Corvettes.

Thanks

Tom

 

I use Mobil 1 75W90 gear lube and two bottles of GM limited slip additive. Mobil 1 gear lube has limited slip additive (Mobil 1 75W90 LS). Whatever you use, I would still use the GM additive, which is per the Mobil 1 web site (if your car uses additive, Mobil 1 recommends OEM additive). The web site has the following info:

Mobil 1 Synthetic Gear Lube LS 75W-90 contains special friction modifiers designed for limited slip-type differentials. In most applications, the addition of special LS (limited slip) additives is not required. For axles requiring the highest level of limited-slip performance, OEM specific LS additives can be added to this fluid.

 

That is funny!!!!