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Is differential noise normal after installing new gears?

I just bought a 2011 mustang v6 today with 86,000 miles for an extremely good price. Mechanically, everything is in amazing shape. The only thing I noticed is the rear differential (with 3.73 gears installed in the past 6 months) is making a howling noise when lightly accelerating. If I’m not pressing the gas it doesn’t make noise. The guy I bought the car from said he had taken the car to have the gears professionally installed and was told by several mechanics a little noise is normal if you install different size aftermarket gears. Is this true?

If it is done properly the differential gears should be as quiet as they were when the car was new. If there is a whining noise under acceleration that goes away on steady state cruise or decelerating, generally the ring gear and pinion gear are not “set up” properly, meaning the way the 2 gears mesh with each other is not correct or the bearings were not properly torqued. Either that or the gears are worn.

It’s impossible to tell here how bad the noise is or if it warrants further inspection. The fix will be anything from a simple adjustment to doing the whole job over again.

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The guy said he had them installed in 2015 and they made noise since they were installed and it hasn’t gotten any worse since then. He also said he asked the people at the ford dealership about the noise when he got the car serviced there and they said it was nothing to worry about. Would there be any consequences to driving the vehicle with the noise? He’s supposedly been driving it for 6 years and like that and it doesn’t effect drivability from what I can tell. If there really was a problem I’m thinking something would have definitely happened by now?

The 8.8 differential tends to be a little noisy naturally. That said, it should be pretty quiet. I have installed aftermarket 4.10 ratio gearsets in 2 different Chevy axles and both were completely quiet, so, No, aftermarket high ratio gears are not all noisy

If the gear has been in for 5 years, you should be able to pull the rear cover and inspect the contact pattern. That will show a knowledgeable mechanic what could be done to quiet the gears.

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Adding to Mustangman’s post:

Do NOT go to a dealer. Go to a local mechanic and ask this question: “Can you check the Rear Gear Lash? It’s making noises and I don’t think it was set up correctly.” Please note: Not every mechanic has the knowledge AND the willingness to do this.

If he says yes, ask how much it would cost and how long it would take - and can he fix it if it is wrong - and how much and how long.

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So why do you say NOT go to dealer? The OP says the gears were professionally installed but that does not mean a dealer installed those gears.

And no, the gears should not be noisy. This is likely caused by the pinion gear not being set deep enough.
Unless corrected (and hopefully not damaged already) failure will occur at some point.

Two or three decades ago a local farmer in my hood installed a new ring/pinion set in his Case tractor. I asked him if he had checked backlash, pinion pre-load, and so on. Nope. He says it will be fine.
Low RPMs and low speeds were not enough. About 7 years later that 1700 dollar set of gears gave up.

I’d also be reluctant to take the car to a dealer for two reasons. 1) the dealer can only address/repair things to factory specs, and this is clearly not a factory spec setup. And 2) if they do find something “wrong”, all they can use is Ford OEM replacement parts. Which tend to be considerably more expensive.

Personally, I’d find a local shop that will open up the differential and give you a diagnosis. You may need to pay them for their time, or I hope you would. It could also be that the differential is low on oil, or the oil needs to be replaced.

It concerns me a little that you say you got a “good price” on the car that may now have (expensive) differential problems. I hope I’m wrong, but you may have discovered why the previous owner sold it for a “good price”.

Good luck.

2 reasons not to go to the dealer. They are usually more expensive and:

“He also said he asked the people at the ford dealership about the noise when he got the car serviced there and they said it was nothing to worry about.”

The OP: did not state that he talked to the Ford dealership;. He stated that the guy who sold him the car said that. Yes, I know it’s difficult to think that someone would lie when trying to sell a car for an “extremely good price”. I suspect the seller knows more than he was actually saying.

The OP also said that those gears were installed in the past 6 months and then later said that the car seller stated that he had been driving on that set of gears for 6 years.
So which is it? Six months or six years…and none of the very high miles on any of my present or past Fords have made a howling noise from the differential. Aftermarket gears should not be any noisier than what the car came out of the factory with with one caveat. And that is IF they are set up correctly…
Heck, my daughter has 2 cars. An '05 Mustang and a '15 RT Challenger. The Ford has 225k miles on it with not a hint of differential noise.

I suspect if this one were opened up and assuming there are no marks on the ring gear yet that an application of Prussian Blue would show a gear mating problem.

It was 6 years he’s had the gears installed and apparently they had been making noise since day 1. He said the noise hadn’t affected inspection (car came with inspection from last July) and I live in Maryland where they’re really strict. Other than the diff noise, the rest of the car was in amazing condition, low mileage and for what it’s worth, the guy I bought it from was retired navy and air force and had a wife and kids. Not saying he couldn’t still be giving me the full story but this is the type of guy that is very honest and everything else he said about the car checked out.

Regardless though I’ll take it to my mechanic later this week and reply again with an update