Hum after rear-end gear change

I'm coming to all of you as the experts.
I've recently had my gears changed in my 1988 5.0L convertible to 3:73:1. (I only have 152,000KM on the car) There's a humming noise
which comes from the rear-end which gets significanltly louder between 60-90 KM / Hr.
I was told by the mechanic that this is a common thing with the gears and should be ignored.
Just wanting to get some thoughts/opinions.
The mechanic I'm dealing with is great and has always done excellent work on the car. He goes over and above what I've asked him to do with little cost.
I'm just thinking that it should be normal as it can become quite annoying.
Thanks in advance, Marc
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wrote:

Just thinking that it SHOULDN'T be normal....sorry
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wrote:

Doesn't sound normal to me... Some gears do whine more than others, but by your brief description I get the feeling you are getting more than normal.... Might want to get a second opinion from a reputable gear shop if you have one in town...
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Then he is not the mechanic you think he is. Properly set up gears do not make noise.

See above. A lot of otherwise competent mechanics do not have the knowledge or tools to properly set the gear lash in a differential. You should take the car to someone who specializes in working on differentials before damage is done (if not already.)
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88 Mustang GT Conv. wrote:

He's correct in that it is a fairly normal occurrence after new gears are installed and not properly adjusted. That's why gear replacement is something I had done, proper adjustment is critical and not very easy.
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Take the car back and tell him to fix it. Gears don't whine when properly installed. Also, after he fixes this problem find yourself a new mechanic.
88 Mustang GT Conv. wrote:

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I had 3.55 gears and trac loc put in my 69 coupe over 3 years ago. The entire rear end was custom built by a shop that builds ford 9" rears all the time for drag racing. So I know they know what they are doing. They test drove it. I drove it home 300 miles. I drive it maybe couple times a month in the summer time. Its made the whining sound ever since they were put in over 3 years ago. They work great. Its more noticable on the highway ofcourse. Im thinking that its fairly normal for aftermarket gears. Ive never had a problem.
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faust snipped-for-privacy@hotmail.com wrote:

I had a set of 3.55s installed in my car and have never heard a peep out of them. This is the way it should be for an 8.8 rear, IMO. Some whine doesn't mean impending doom but it is typically an indication that something is off to some degree. It is easy to tell if gears are not wearing properly by looking at the wear patterns on the teeth.
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faust snipped-for-privacy@hotmail.com wrote:

I have to wonder if the difference was the rear ends. I've never had an 8.8 do what you are talking about, but the 9" rear is different with the removable snoot, correct? The only car I have ever owned that made a noise from the rear end was a 73 Charger that had the 8 3/4 rear that also has the removable snooty on the front.
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Michael Johnson wrote:

The common thought, but I would be reluctant. I am very very picky about by whom and how my Mustang is worked on (all my cars really) and I'm not sure I'd want a half-ass like that going into it a second time. What *I* would do is ask people that know, car clubs or at a race track and get it readjusted then dispute the charge on my credit card. Go back to the guy with proof that proper alignment fixed the problem and and make him cut the bill in half at least. If he fails to comply I'd tell him I was disputing the charge and taking him to small claims. He will probably change his mind as long as you have proof that realigning the gears fixed it and that is actually true. One has to assume you could get a defective set of gears that is noisy and maybe it will go away, maybe it won't. No car that I've had a gear replacement done to made noise afterward.
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What brand were the gears? Ford gears have earned a reputation (at least among the installers I know on- and off-line) for requiring the least amount of adjustment and resulting in the least amount of noise after installation. I know a couple of local installers that won't touch any other brand because of the difficulties they've had in the past with other aftermarket gears.
A little whine is not uncommon because of wear and tolerances in the carrier. Sometimes it goes away, or at least gets quieter, as the gears wear/break in.
However, something described as a "hum" doesn't sound right to me. The worst noise should be an almost subliminal high pitch whine. More than that means it's not shimmed right or something isn't meshing well.
Dan P49Y83+
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I had a rebuilt engine put in, went to solid mounts, new clutch, and they changed out the oil in the rear end. And ended up with a lot more drive train noise around 65 to 80 MPH, very noticeable, in any gear with clutch engaged. After driving it for a few months, it is gone. I think the angle of engine to drive shaft was off. (only like 4 degrees or so total) If it was just gears, it should not have a resonance 60 to 90 km/hr, should be noisy proportional to speed, seems like the tires/driveshaft would have to be involved too. Let us know what you fine out.
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88 Mustang GT Conv. wrote:

Let me guess. If you're cruising at speed with a light throttle, and then lift your foot off the gas, the noise goes away (or gets significantly quieter).
Sounds like it wasn't set up properly.
Cheers!
zk
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Pretty much what everyone in this thread has said.
The best way to go about it is to get a complete used differential/axle with the ratio you want. Ford rear ends take a very long time to wear out. The only one I ever had that made any appreciable noise was an 8" that had over 600,000 miles on it. The 8.8 in my '71 Galaxie has over 240,000 on it, and is quiet as a mouse.
On the other hand, mechanics who don't understand how to set a rear end up properly (and they are few and far between now) can cause a lot of trouble in a heartbeat.
I read over the procedure in my 1969 Chilton's book, and it isn't anything I would want to tackle.
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wrote:

This is exactly what happens. The noise is less significant (although not gone) when the throttle is released.
To answer some other questions. The gears were provided by the mechanic and I don't know the brand. I'm not a car buff (I like a nice car but know little about them) and therefore rely heavily on other people's knowledge experience.
The car is an automatic and I have 2" flowmaster exhaust (which make enough noise on their own) and the humm overpowers them easily at that speed range (with the top up or down).
Thanks to everyone, I will contact the mechanic and discuss with him.
Marc
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88 Mustang GT Conv. wrote:

I think I'd get another mech to check it, someone recommended by people from a track. Also 2" exhaust? THe stock Mustang pipes are 2 1/4" the upgrade is usually 2 1/2 or 3"
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Sry...miss typed and didn't verify. I do have 3" pipes :)
Marc
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On Wed, 30 Jul 2008 05:29:27 -0700 (PDT), "88 Mustang GT Conv."

Similar problem. Had the differential checked as a speed shop. Not a thing wrong with the unit. When they put it back together, no sound. The only change was the posi lube that gets mixed in. The speed shop said to only use the Ford posi lube. Have not had a bit of problem since. Nice and quiet. As I recall, and I can tell you what it calls for, but they used 2 tubes of the Ford posi lube.
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guessing you are talking about the "friction modifier"... helps the clutches slip when you go around corners... without it they'll chatter a bit..
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John S. wrote:

"Whale oil" There are synthetics of that stuff now by Amsoil and Synergyn
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