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Anything to quiet a noisy Subaru front differential? #5290274 12/10/19 10:15 AM
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anarchyx34 Offline OP
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'05 Legacy 4EAT automatic with 114k miles. Car is new to me as of earlier this year and I noticed a slight whistle/howling noise from the front diff while cruising at 60mph. Goes away if I decelerate or accelerate. Only makes the noise in that "sweet spot" of maintaining constant speed on flat ground. Sometimes it's louder or quieter and I have yet to determine if temperature has anything to do with it.

Fluid seemed to be in good, clean shape with nothing of concern on the magnetic drain plug. Just the usual black/gray sludge you'd usually find on any diff with some miles on it. Ring and pinion also look good verified with a borescope stuck down the dipstick hole and drain plug. No weird wear pattern or chipped teeth. Everything visually looks perfect. I'm guessing the pinion bearing is the problem here. Not a particularly easy thing to replace on these and I'm guessing I've got some time on it. It hasn't really worsened in the 3000 miles I've put on the car. I just want to quiet it down a bit. I refilled with a conventional 75w-90 but I'm wondering if there's something I can add that will help. I know bandaids are just bandaids. I'll get around to fixing it one day but that may not be for another year or two. The noise is a bit annoying on extended drives.

Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290320 12/10/19 11:19 AM
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Cujet Offline
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Generally, viscosity is what helps quiet noisy gears.


People who count on their fingers should maintain a discreet silence.
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290350 12/10/19 11:52 AM
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Rand Offline
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you could try some 75w140.


2019 Jeep Cherokee Trailhawk 2.0T
2020 Hyundai Elantra "Value Edition"
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290381 12/10/19 12:26 PM
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philipp10 Offline
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Are you sure it's not a wheel bearing? They are known for that, I had one whine for a long time until I figured it out.

Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290416 12/10/19 12:56 PM
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askani79705 Offline
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Call Redline.

Last edited by askani79705; 12/10/19 12:58 PM.

2019 Ford F150 NAPA syn 5w20 Motorcraft Filter
2000 Chevy 2500 Silverado NAPA conv 5w30 AC Delco Filter
2012 Impala MileSYN 5w30 AC Delco Filter
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290499 12/10/19 02:18 PM
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nthach Offline
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Subarus(and Audi quattro) are sensitive to tire differences - the quickest way to kill a Subaru is mismatched tires. All four tires should be the same brand/model/size and be within a 5-10% difference across the set for treadwear.

Check to see if all the tires are the same brand/model and then work up to wheel bearings. The diffs/transfer unit in a non-turbo Forester/Impreza and Outback/Legacy isn't anything special - the front and rear diffs are standard open units and the center diff is a viscous unit.

Re: Anything to quiet a noisy Subaru front differential? [Re: nthach] #5290505 12/10/19 02:23 PM
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Rand Offline
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Originally Posted by nthach
All four tires should be the same brand/model/size and be within a 5-10% difference across the set for treadwear.

Check to see if all the tires are the same brand/model and then work up to wheel bearings. Tthe front and rear diffs are standard open units and the center diff is a viscous unit.


3/32 is the published spec for treadwear differences.

Also your info is incorrect.. the center diff being a viscous unit is only true on some manual transmissions.
automatics used a MPT clutch mostly.


2019 Jeep Cherokee Trailhawk 2.0T
2020 Hyundai Elantra "Value Edition"
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290709 12/10/19 06:05 PM
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anarchyx34 Offline OP
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Definitely not a wheel bearing. I'm 100% sure of that. Also I'm running with the AWD disabled (FWD fuse) and it makes no difference. I took a video. It's a bit hard to hear but it's clearly audible from the 30 second mark.

https://drive.google.com/file/d/18uklD_r-n3sQq9AzzdFsCo3pNwN96T-_/view?usp=sharing

Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5290984 12/10/19 11:35 PM
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BlakeB Offline
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Any chance you’d try this as an experiment to see if it helps any?
https://lucasoil.com/products/engine-oil-additives/heavy-duty-oil-stabilizer
[Linked Image]

Re: Anything to quiet a noisy Subaru front differential? [Re: Rand] #5290989 12/10/19 11:53 PM
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KneeGrinder Offline
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Originally Posted by Rand
Originally Posted by nthach
All four tires should be the same brand/model/size and be within a 5-10% difference across the set for treadwear.

Check to see if all the tires are the same brand/model and then work up to wheel bearings. Tthe front and rear diffs are standard open units and the center diff is a viscous unit.


3/32 is the published spec for treadwear differences.

Also your info is incorrect.. the center diff being a viscous unit is only true on some manual transmissions.
automatics used a MPT clutch mostly.



I believe the newer 3.6R's use a viscous unit, and they are fairly common now that they put the motor in two of their more popular vehicles. The viscous unit may have gone bye bye with the change to CVT!


As for the tire spec, the data I have is roll out, but my tires are probably bigger than most. My data is for 255/55/18, 0.125 side to side is max acceptable difference total diameter, on the same axle, same brand tires all four corners. Front to rear 0.250 max, Larger diameter need to go in the front! "Subaru Tribeca"

Also, does mention measuring tread depth but is vague and says straight up 1/8 inch under wear in any tread depth from the other tires is out of spec. 3/32's Put them in front if a pair match.

I measure my tires closely every winter when I put my snows on, and its time to do that this week I believe!


08 Tribeca 90k PUP 5w30
06 Accord 137k PP 0w20
03 Acura CL-S 90k M1 AP 5W20
02 Acura TL-S 241k ST 5W30
02 Accord EX 155k PP 0W20
02 K3500 99k M 1 ESP 5w40
00 F350 139k M 1300 15w40
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5296447 12/16/19 09:56 PM
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ToadU Offline
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Didn’t your Subie come w a stereo? If yes, turn it up a bit. I prefer hard rock. banana

Last edited by ToadU; 12/16/19 10:23 PM.
Re: Anything to quiet a noisy Subaru front differential? [Re: anarchyx34] #5298185 12/18/19 03:27 PM
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bluesubie Offline
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If you've ruled out a bad wheel bearing, and a difference in tire diameter, I'd try some Amsoil Severe Gear 75W-110 or 75W-140.


'20 Subaru Outback Onyx XT | FF Idemitsu 0W-20
'04 Subaru Forester 2.5XT | Edge 0W-40
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