January of this year I had noise coming from what I thought was the rear Diff of my 1999 F250 7.3, Auto, 2WD Super Duty with Limited Slip. The noise comes at that point where there is no load on the differential, once you give it some gas it goes away.. I only now have 72 K on the truck. At the time of the repair I had around 60K. The Shop I took it to was convinced it was the Front wheel bearing...Well, after they replaced the front wheel bearings it was found to be the rear diff. Yep, had to pay for those front wheel bearings that I did not need to be installed. I had them pull the rear diff and put in new carrier, pinion and axle bearings. One of the carrier bearings were toast. No reason for this to happen with such low miles except for the people I bought the truck from at 27K in 2005 let it sit a lot? This the low miles.

Now when they did the rear diff they told me my ring and pinion gears were fine. Ever since I picked the truck back up I still get a little howl or hum from the rear diff. They said part of the noise could be amplified because I have a camper shell? Also even though the ring and pinion gears were good, when it got setup by them the ring and pinion would run in just a little different spot maybe cause a slight noise? They used a special synthetic oil (cant remember which now) that had the Limited Slip additive already in it? So they did not need to ad any additional additive.

Now did they just do a poor job setting up my rear diff or is this the way it goes? Can anything be done now to quiet things down? Ie.... Different diff oil? I know I cant use a Red Line type Syn with Shockproof in it because of the limited slip.

Any input would be appreciated.

 

I would add a little of the friction modifier (the LS additive) and see what it does. Sometimes you can get away with none with the real good synthetics and sometimes you still have to add a little. I would try that 1st. Im sure someone else here will chime in and might have actually had this problem before!

 

X2. I had a noise after changing the rear dif oil, I used Royal Purple synthetic with no modifier.

After a while i added the modifier and the noise went away after a couple hundred miles.

If you do try the modifier, make sure you wear rubber gloves, because it smell terrible and takes forever to wear off your hands. Trust me Good Luck.

 

Smells good now Mark We serviced some bushhogs here back of this and we got real nice and messy .

 

I added the modifier and I think I hear some improvement. The parts guy at the dealer said all I needed was one bottle. I think they are 4oz....Would it hurt to add another?

 

You used the whole bottle already?? Id ask what brand they put in your diff. Doesnt sound like too good of a synthetic. Ask them what weight they put in it as well. I hope they put 75-140 in it. I didnt use any modifier in mine when I changed my fluid. I bought Mobil right off the shelf at Auto Zone. 14.99 a quart! I havent read about anyone needing that much modifier, but then again who knows!

Just make sure you add it in small quantities, test it, add, test it etc.

Personally I would change it myself just to be on the safe side. At least when you do it you know you did it right!

 

Tristan made a good point.
You may be chasing your tail if they used the wrong oil.

I remember it took a couple hundred miles after i had the 4 oz bottle of modifier for the noise to stop. may need more time to work.