Rear End Whine on Acceleration and Deceleration

RAU03MACH

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always used Ford Racing gears never had any bad luck with them
im getting ready to do my 98 gt with a set of 410
 

Notthenow

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Yep, machining consistency and style of gear grind makes them easy to install and quiet! Always keep track of the original pinion shim if possible!
 

b1pig

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I know that the heavy cover is to reinforce the case while in motion, but I haven't heard of a case warping. I mean axles bent, sure, and case distorting under load, but it would stay distorted when unloaded.....

The preload diff cover helps prevent issues with the ring gear being "pushed" away from the pinion gear under heavy loads. Most common in offroad rigs, which seen very very high torque loads at very low tire speeds. Common also in performance cars where owners just want additional insurance.... you know for burnouts, drifting, general hoonage.

All metals have a fatigue limit. Even if you never exceed a designed limit, repetitive instances where you come CLOSE to it can still result in fatigue failures. Yes. The differential can deform (bend) out of spec if it is damaged in some way. It is possible you could not even see it or be apparent to the naked eye. Like I said previously, unless you stripped the entire housing and put an alignment bar in the housing, you may not even being to see the failures.
For reference... the tools exist....













All of this aside, in reference to the original issue by the OP....
With the noise continuing after multiple refits... there's an issue here.
 

b1pig

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Shop wants to claim the bellhousing is warped in the pinion area.

I would like to point something out... not just to be "that guy".... but....



A "bellhousing" doesn't have a dang thing to do with a differential. :)

If "diff housing" was the intended statement, I understand.
 
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Thyme

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I would like to point something out... not just to be "that guy".... but....



A "bellhousing" doesn't have a dang thing to do with a differential. :)

If "diff housing" was the intended statement, I understand.
My bad, definitely meant the diff housing. Its been a rough couple of weeks dealing with this.

All metals have a fatigue limit. Even if you never exceed a designed limit, repetitive instances where you come CLOSE to it can still result in fatigue failures. Yes. The differential can deform (bend) out of spec if it is damaged in some way. It is possible you could not even see it or be apparent to the naked eye. Like I said previously, unless you stripped the entire housing and put an alignment bar in the housing, you may not even being to see the failures.
I'd be amazed if I managed to warp the housing with this car. The worst its seen with me is a spirited driving on backroads and the occasional skid in the rain. I don't race the car at all. The whole reason it blew out the first time was because of me botching a pinion oil seal job. Managed to drive it about 600 miles before it started screaming and I limped it to a shop. Then it managed another 2000 miles in one go on that rebuild, whining the whole time.

Plan right now is to get in touch with a more performance oriented shop and get a second opinion on all of this. If the housing really is warped... I'll probably choose to live with it, and keep an eye on temps + changing the the diff oil along side the motor.
 

Notthenow

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I would guess the axles wouldn't go in, or would be a really tight fit if the housing was warped and causing a misalignment. We put a big Buick 8.5 axle in my son's 1996 Z28. The axles were in the carrier, but were tough to reinstall. It was easy to see the axle tubes were bent. A trip to a axle shop was needed to straighten them. They apply heat to the tubes to get them to shrink in the proper direction. The alignment bar verifies.
 
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Thyme

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Update: I've been doing break in drives. Haven't seen temps higher than 190f anywhere on the diff even after 70-80mph drives. stops whining under load around 80 too. However, when I coast from that speed it makes an extremely loud whine. Its warrantied for a year so I think I'll stick with it until it blows. I'll also be checking the mesh pattern at the end of the break in period.

Something else of note is a kind of resonating vibration around 70mph. It comes and goes in about 2 second intervals. doesn't happen at all once you're around 60, and I haven't tested it at less legal speeds.
 

cobrajeff96

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You're unwittingly trying to diagnose a whole lot of things all at once. Vibes can come from many sources in the suspension and/or drivetrain. My car also once had the same pulsing vibration at highway speeds and beyond. My personal opinion, but if you ever look at and hold a wheel hub/bearing assembly from a modern German car and compare it to ours, you'll be amazed at how much thicker/beefier/heavier they are. A lot of frequency is coming through them and being generated by them. And our SN95 floor pans are definitely soda can thin compared to most Euro vehicles and the more modern American ones too. I think some vibes in an old Mustang are just inevitable. But the engine soundtrack should take care of that.
 

lwarrior1016

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Update on the rear: Shop redid the work under warranty, with new gears (3.55), bearings & races, shims, and crush sleeve. I noticed the accel and decel whine immediately upon leaving the shop. Biggest difference was that both noises became apparent at lower speeds than before, and the decel whine is even audible below 20mph. Driveshaft still shakes along its axis of rotation when slipping the clutch in first.
Shop wants to claim the bellhousing is warped in the pinion area. Claimed the preload was set to 25inlbs with 0.011" of backlash. I'm just about ready to say to hell with it and just drive on it as long as it isn't making glitter.
That pinion preload sounds a little tight, and the backlash sounds a little loose. Especially on a new set of gears. I always try to set preload around 20inlbs and backlash as close to .008” as possible, if not a little tighter. As things wear, the backlash will loosen up. If it’s loose to start with, it’ll be out of spec in no time at all. Loose gears make noise. And with a really tight pinion preload, it will kill pinion bearings.
 

Notthenow

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Did there used to ne a rule of thumb on when you heard whine, as accel vs. coast to help determine source? As in pinion depth vs. backlash?

That vibration rhythm sounds like driveshaft to me. Like brum, brum every 2 seconds.
 

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