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Old 08-08-2006, 11:37 AM   #1 (permalink)
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Default Help diagnose rear end noise

Over the past few days I have heard rear end noise. It is most noticeable at highway speeds, starting about 55mph.

There are two things I can think caused this. First, I checked the tire pressure all around this morning and noted the driver's rear tire was 10psi low (34psi norm) which was surprising because the last gauge check was 3 weeks ago and I do a visual check daily and it never appeared to be deflated.

Secondly, I made a high speed run about a week ago, before the noise started. It was about 5 miles at a sustained 110-120mph. Normally I cruise anywhere from 70-80mph.

Now I've had both a GM f-body and a Jeep ZJ, so I know what bad diff bearings and such sound like and it's not that. This noise is constant and speed dependent, NOT accel/decel dependent, and more of a moan than a whine. Any help in figuring out what it is would be very helpful. I can change the fluid this weekend when I change the engine oil, but that's 300-350mi. away.
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Old 08-08-2006, 11:44 AM   #2 (permalink)
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Most of my rear end noise is because I fart too much.

How certain are you that it's coming from the rear? Are you driving an auto or MT?
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Old 08-08-2006, 12:00 PM   #3 (permalink)
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My first guess would be the common rear diff howl that plagued the 04 & 05 FXT's. Do you hear it on light accel around 55-65 mph and while coasting?

My other guess would be one of your rear wheel bearings going bad.
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Old 08-08-2006, 08:00 PM   #4 (permalink)
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I have narrowed it down to the rear diff after having someone else drive while I crawled around in the back. It does grow moderately louder when decelerating from 80 to 60.

However during this process, I discovered another noise.

This one is a rapid clunking over road blemishes. It sounds like something is loose in the suspension? I need to get the car in the air this weekend and investigate these things.

I find it odd that in the span of a few days both these things develop..
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Old 08-09-2006, 06:47 AM   #5 (permalink)
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I had the same whine/moan issue on my FXT last summer... I had about 7,000 miles on my 05 XT 4EAT and around 55mph it would start whining/moaning on the highway... I braught it back to the dealer who held onto it for a week before returning it with a new Diff... and that did solve the problem
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Old 08-09-2006, 07:05 AM   #6 (permalink)
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This might be off-topic.

Would it make more sense to go with aftermarket Diff if you no longer have warranty? What are the options?
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Old 08-09-2006, 07:25 AM   #7 (permalink)
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A cheaper way to get rid of the diff whine is to simply put better gear oil in the rear diff. case. That solved the noise for me 40,000 miles ago, and have not heard it since.

This should work for most individuals, and 20 minutes and $10 is a lot better than a week without a car, and $1500 for a new rear differential.

Unless your differential is really shot, just try switching oil to a nice synthetic.
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Old 08-09-2006, 08:07 AM   #8 (permalink)
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I'll be changing the fluid this weekend.

Thanks for the suggestions.
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Old 08-09-2006, 09:06 AM   #9 (permalink)
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Quote:
Originally Posted by F1 Rox
This might be off-topic.

Would it make more sense to go with aftermarket Diff if you no longer have warranty? What are the options?
This is a good suggestion for someone that is out of the warranty period. Which aftermarket diff are you talking about?
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Old 10-27-2006, 12:51 AM   #10 (permalink)
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Default Me as well...

I just started noticing the same thing in my 99 S Turbo. At first, I thought it was the bass line in a track I was listening to! But when it didn't modulate like the rest of the instruments I realised I had a problem. It's a low-pitched moaning sound that is loudest at about 55mph/90kmph, and curiously enough it goes away if you accelerate over 70mph/110kmph... If I accelerate slowly up to that speed in 5th it doesn't always appear, but if I accelerate hard in 3rd, then 4th, it's quite loud.

Would you recommend trying the gear-oil change before having the diff replaced? I certainly don't want to buy a new diff!

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Old 10-27-2006, 09:07 AM   #11 (permalink)
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I just changed the fluid and it worked out fine.

Here's a tip: Once you've got the majority of the fluid to drain out, get your finger in the drain hole and start scooping out what's left at the bottom. I found quite a bit of diluted metal in mine.
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Old 11-08-2006, 07:18 AM   #12 (permalink)
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Fluid changed, and it seems better, but still happens occasionally, particularly on a section of road with a series of roundabouts joined by dual-carriageway. I am beginning to suspect a wheel bearing as well, as straight line acceleration seems to be fine, but acceleration off a roundabout (lots of right-left steering input) causes the noise to return, but not while turning, just afterwards as I pass 90 kmh in third (or second ). Or maybe the noise will gradually fade away as the oil gets to work; as mentioned by Aleksey, I think, on another thread. Ah, I dunno.
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Old 11-08-2006, 09:41 AM   #13 (permalink)
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Ahhh I am not alone. I've tried the diff oil route and its still there.
So I've tried recording it on my Nokia phone but can't work out how to post it so anyone can hear - its not the best recording ever but gives an idea.
It's like the diff noise is being amplified somehow but only in the car no noise when I drive past someone listening or hanging from the window.
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Old 11-09-2006, 02:27 AM   #14 (permalink)
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How long has the noise been there in your case? If it is just a noise that can exist for years, and doesn't mean the car is about to grind to a halt, then I could deal with it much better; probably by turning up the hifi!
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Old 02-21-2007, 08:33 AM   #15 (permalink)
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The noise is gone. It was actually a rear wheel bearing. The reason it was only audible at certain speeds is because, well, I have no idea... It became audible at most speeds as it got worse. I think I just picked it up very early because I drive a long trip once a week for work, really heating up the bearings (it includes a "mountain stage" of backroads a bit like those intended for the upcoming Irish WRC leg, which I love, so poor old Forester gets a hard time), and the last section of the trip involves sprints on dual carriageway between a series of roundabouts (which I can't resist, of course. Pedal to the metal all the way!). The hard cornering around each roundabout, followed by the hard acceleration through 3rd gear, seemed to put just the right/wrong pressure on the bearing, and then it would start moaning in protest. Eventually, though, I could hear it most of the time. Mind you, even the mechanic sitting beside me as I demonstrated the noise thought it sounded like the diff, so I guess you never know until you get the car up on the ramp.
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