Rear end noise?

I've got a noisy rear end. Maybe a problem, maybe not, but I need your help. 

The culprit is the 1997 V6 Mustang that got dropped off at my house. It's been sitting for at least three years, and the differential is full of something brown/grey and horrible. It's noisy at steady speed and pretty much all the time with the exception of maybe just slightly holding off when transitioning from acceleration to deceleration. 

I assume my first course of action is to pull the diff cover (becasue there's no drain plug, of course) change the fluid and inspect for any super obvious signs of horribleness, but where to do I from there. How do I figure out if it's bearing of the gears of the differential itself? 

I *think* this would have an 8.8 open rear. Super easy and simple to rebuild in the car. Pop the cover and see what you can see. Rebuild kits are cheap and plentiful if you need one. A clever and resourceful fellow like you oughta be able to source some gauges to check bearing preload and gear backlash.  Ford Racing sells gear kits and bearing kits and everything plays well together. You'll need (if rebuilding) a backlash gauge (is that what those are called?) and an extra set of hands helps. It is an easy weekend project and a great opportunity to add some extra wheel torque through gearing. Just saying.

My 96 came with a 7.5 but it's the same procedure and even some of the parts match.  When did they stop putting the 7.5's in the mustangs.

In reply to akylekoz :

The 7.5" was used right up until the end of the straight axle in Mustangs behind the V6. 

Whine or rumble?  Wheel speed, or driveshaft speed?  Noisy on coast or drive?

Whine is gear related, rumble is bearings.

Wheel speed could be an axle bearing, but more likely a side bearing.  Driveshaft speed is pinion bearing, and is probably the most likely problem.

Coast or drive implies a gear mesh issue.  Coast and drive implies a bearing problem. 

If you have a rumbly bearing, unless it is extreme, or the bearing has allowed the pinion to actually move around, the gears are probably OK, unless you turn the radio down and listen really carefully.

If you do have a bearing that has lost the surface, it takes forever to clean out all the steel flakes.  Remember to clean the axle tubes, and if it doesn't have one, add a magnet. 

Unless you have a 2 speed rear end, arent wheel speed and driveshaft speed directly linked? Well, unless you're driving a continuous turn..

Speed related. I would call it a whine. Not high pitched like a front wheel bearing (the only one I have experience with) but lower than that. Not continuous on accel/decel. There is a bit of a gap when things are neutral which is why I suspect something gear related, but maybe unloading the pinion bearing would make it stop for a second? I've never done more than change the fluid in a rear end, so this is all new to me. 

I had rear end noise once, it was diet related.

I would just go get another rear end / axle from the junk yard and swap the whole thing as a unit.  Maybe put new axle seals.  Definitely do fluid and inspect the internals before installing.    Calipers and rotors swap over so if they are decent you could save some $$$ there.  I would probably look at new hard lines and the flex line but all that is simple and easy with the axle out of the car.     

ultraclyde said:

Unless you have a 2 speed rear end, arent wheel speed and driveshaft speed directly linked? Well, unless you're driving a continuous turn..

They're linked but the drive shaft turns at a much faster speed (As an example; 3.73 times faster if you have 3.73:1 gears) so to an experienced ear it's easy to tell them apart.

And trying to send the "experienced ear" out over a forum post is pretty much impossible.

Streetwiseguy said:

And trying to send the "experienced ear" out over a forum post is pretty much impossible.

Yup, but in the tradition that's as old as the internet itself we're going to try.  Based on Seth's input so far I'm thinking gears and if that's the case then the whole rear end needs to be setup which will mean replacing pretty much everything else that might cause the noise allowing all of us to feel that our guess was correct.

Actually Seth, perhaps you could record the noise?  If it's loud enough you might be able to get it with a cell phone video.

Sounds like solid article material to me. 

grover said:

Sounds like solid article material to me. 

Another part in the continuing series of "mazdeuce takes apart things he doesn't understand" 

Super fun. 

APEowner said:

Streetwiseguy said:

And trying to send the "experienced ear" out over a forum post is pretty much impossible.

Yup, but in the tradition that's as old as the internet itself we're going to try.  Based on Seth's input so far I'm thinking gears and if that's the case then the whole rear end needs to be setup which will mean replacing pretty much everything else that might cause the noise allowing all of us to feel that our guess was correct.

Actually Seth, perhaps you could record the noise?  If it's loud enough you might be able to get it with a cell phone video.

If I knew anything at all about music, I would say something like, "If it starts at middle C at 20 mph and is up to High F by 40, its a pinion bearing" but since I took exactly one piano lesson when I was about 6, and whined so hard my mother (Who loved playing the piano, and had a real talent for it) allowed me to quit, I can't do that.

In reply to APEowner :

I'm up for throwing my phone in the trunk and seeing what gets recorded.
I'm imagining a kidnap victim describing their experience to the police: 

"the car had a noisy differential, speed related, noise under acceleration and deceleration" 

"Did the sound follow the rear end or the driveshaft speed sir, think carefully, it's the key to cracking this case"

I would totally read an article about a mystery rear end noise. 

If I were doing it, I would pop out the axles and give the bearings a spin and see how they feel. Those aren't it? Four bolts and the diff carrier comes out. Now you can feel those bearings and inspect the gears. Those all good to? All that's left is pinion bearings. You can get a good feel for those now with them installed because they aren't driving anything.

Actually if it were me I would get an 8.8 from a mustang gt and swap in some fun gears.

99.9% sure it's 7.5 rear with 3.08 gearing.  I'm with Streewiseguy, more than likely the pinion bearing.

Video of riding in a Mustang trunk uploading. I do not have good upload speeds, it's going to take a while. 

EastCoastMojo said:

I would totally read an article about a mystery rear end noise. 

Only if it's laden with innuendo though.

Pretty sure its just like the 8.8 as far as how its put together (not really as far as parts)

Its got nothing on a mercedes engine. can do a total rebuild for cheap in an afternoon. 

I build diffs.

For the time and effort I can almost guarantee it'll be faster and easier to just get another complete rear axle from another car. They are not rare.

That said, as far as the noise goes, pinion bearing failure is common. Your description isn't really consistent with pinion bearing failure or carrier bearing failure.

Pinion bearings will whine under certain accel or decel loads. Give it more or less gas and the noise will go away or quiet considerably.

Carrier bearings will make noise at all speeds.

Same with gears but will tend to change in pitch/tone with varying loads.

Here is a gear set up how to I did LONG ago. Main differences: The Mustang rear has crush sleeves for pinion preload, not shims. Easier to set up(and easier to screw up if you go too far).

http://www.jeepforum.com/forum/f19/regearing-marinejrm-s-xj-d44-992471/#/topics/992471

Download the Photobucket fix plugin for Chrome and all the Photobucket pictures will show up for you. Probably need to do it on a PC. 

Also the axle shafts will be held in with c clips not bearing retainers on the ends.

Flatulence stinks. 

mazdeuce - Seth said:

In reply to APEowner :

"Did the sound follow the rear end or the driveshaft speed sir, think carefully, it's the key to cracking this case"

If you crack the case then you should definitely just replace the whole thing.