[jthomson]

My '94 cab has really been a good car, having only given me small problems at worst. So... a couple weeks before I put it away for the Winter, a sound appeared. A lower-mid frequency hum that a freind and I traced to the rear right side. It's speed dependent pitch-wise and the timbre doesn't change according to the road surface. Thought it might be the tire but they make a different, higher-pitched sound. They have less than 7k miles, as well. Today, when I took it out for the first spin, the hum was still there, perhaps a bit more pronounced. Would this be a wheel bearing? My experience with wheel bearings has always been with the high-pitched variety and the wheel falling off and rolling down the highway and you praying that it doesn't cross over the median and kill somebody. Yes, this happened... Any ideas? Get it up in the air and run a few gears? Or just replace the bearings as a preventative measure? As always, thanks!

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[Langley968]

Wondering if it is the (in)famous pinion bearing...

Those with experience will have to chime in. My 968 had the issue dealt with by the prvious owner.

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[Cloud9...68]

I thought about that, too, but the OP says it's coming from the right rear, which kind of argues against it. The tell-tale sign of a failing pinion bearing is that it sounds like a jet engine, and it gets louder as the speed increases.

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[ds968]

Replacing the REAR wheel bearings is a colossal pain in the a**, not sure I would do that just as a "preventive" measure , nor as the first step in the process of elimination in attempting to identify what is causing the hum you're hearing . The pinion bearing failure does indeed have a very distinct whine which is almost identical to a jet engine sound after the plane lands and is taxiing on the runaway . And in my case it was most audible as the car decelerated, slowly coming to a stop, it did not get louder at higher speeds. The pinion sound will be there whether in gear or in neutral., though I suppose the same is the case with a bad tire or a bad bearing.. And sounds are very tough to describe and interpret in writing, but to use some common terms, tires generally howl, bad bearings generally whirr, but no idea what causes a low-mid frequency hum.

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[Rob in Oz]

I experienced the same type of noise when I bought my 968. Read all about the pinion bearing issues but the noise was low pitched as the OP described, not a jet aircraft or dentist's drill type of noise. Replaced the rear wheel bearings, both sides - noise gone! Much cheaper than pulling the trans to replace the pinion bearing, which would have had a disappointing result anyway. Yes the bearing removal is a PITA, and well-left to someone with the right equipment.



Rob

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[bombfactory]

One way to help determine if it's a wheel bearing is to find an empty road somewhere where you won't freak people out, get up to 40 mph or so and slalom left and right a bit. If you suspect it's a right side bearing, the sound should get louder as you turn left (more stress and weight transferred to the right side), and softer as you turn right.



If the sound remains constant as you turn left and right then it's probably transmission related (possible pinion bearing - though that sound is a roar, not a hum). Good luck - hope it's something minor!

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[Bulti]

Could a wheel bearing also make a knocking sound? One that speeds up along with the speed of the car?

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[Rob in Oz]

[quote name='Bulti' timestamp='1396286990' post='156450']

Could a wheel bearing also make a knocking sound? One that speeds up along with the speed of the car?

[/quote]



No - that would more likely be a worn CV or loose bolts securing the driveshafts to the trans or hubs...



Rob

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[Bulti]

CV joint. Check. Probably one more thing to add to my to-do list.



Thanks Rob!

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[fleetwood]

Is the sound present when not moving, as well?

If so, might be the electric fuel pump. I had one that made a low-freq droning sound from the day I bought the car.

Became a little louder over time -- replaced the fuel pump, check valve and filter, and no more sound.

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[Bulti]

No sound when standing still, I will investigate when my new garage is finally finished and my lift is installed.



Maybe the fuel pump is what's causing the OP's noises?

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[speedysprocket]

I just did the rear wheel bearings in my 944. I had a low pitched drone that kept getting louder the longer I drove, and louder the faster I went. Was getting rather loud above 80 too. Had a Porsche guy do an alignment and after driving it, he told me wheel bearing.

So I bought new bearings because I wasn't going to do just one, and I bought the big slide hammer set from Harbor Freight. Oh, and brought the car to a local garage to have them break the spindle nuts loose since I broke two craftsman breaker bars trying to do it myself. (They didn't charge me for the 5 minutes to put the car in the garage, spin the nuts off, and snug them back on, and pull the car out of the garage.)

Did the first side an about 1 1/2 hours, the second side maybe just over an hour. All the noise was gone and after inspecting the bearings, one side of one of the bearings had a lot of pitting in both the inner and outer race.

But as for your concern with the wheel falling off, pretty sure it won't happen. I don't think the spindle would fit through the trailing arm even if the bearing completely failed! I was warned though that a bad bearing for too long can start to wear the spindle...

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[968Syncro]

How many miles are on the car? Wheel bearings shouldn't fail unless higher mileages.



Try to rule out the tires before assuming it is the wheel bearings. See if you can try some other tires first, maybe swap the left and the right. Old tires can make a lot of scary noise, perhaps if they are older than ..... say .. 5 years.



And definitely do the "swerve" test like bombfactory said. The sound from failing bearings will vary as weight changes (left turn vs right turn), assuming it is possible to hear accurately from the drivers seat. Maybe try to get up to speed, put it in neutral to reduce other noises and then swerve side to side. But this test works better for front wheels.

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[Cloud9...68]

[quote name='968Syncro' timestamp='1396382375' post='156489']

Maybe try to get up to speed, put it in neutral to reduce other noises and then swerve side to side. But this test works better for front wheels.

[/quote]

Or, to better stress the rear bearings, he could do this maneuver in reverse <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" />

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[jthomson]

[quote name='bombfactory' timestamp='1396284540' post='156448']One way to help determine if it's a wheel bearing is to find an empty road somewhere where you won't freak people out, get up to 40 mph or so and slalom left and right a bit. If you suspect it's a right side bearing, the sound should get louder as you turn left (more stress and weight transferred to the right side), and softer as you turn right. If the sound remains constant as you turn left and right then it's probably transmission related (possible pinion bearing - though that sound is a roar, not a hum). Good luck - hope it's something minor![/quote]



Went slaloming today. You called it, except it's the left bearing. The hum actually dies completely on harder left turns. It just sounded like the hum was coming from the right side in the drivers position, but if I listen between the window and seatback it's much louder. Hopefully my local garage can handle this cause the nearest Porsche dealer is quite a ways away. They've done a great job so far! Thanks gang!

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[Cloud9...68]

Glad you found it. And while it sounds like a rear wheel replacement isn't exactly cheap, at least it should be a lot less than a pinion bearing replacement.

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[MCL968]

I try to replace those kinds of things in pairs. Perhaps you should consider doing both. My logic has been whatever happened to the left has also happened to the right, and if it was sufficient to wear one out, the other may not be far behind. You may get a better price on two.

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[ds968]

Does a <acronym title='Limited Slip Differential'><acronym title='Limited Slip Differential'>LSD</acronym></acronym> allow for a more even wear of the two sides ( whether faster or slower ? ) , as opposed to cars without a <acronym title='Limited Slip Differential'><acronym title='Limited Slip Differential'>LSD</acronym></acronym> where one side might wear out faster than the other ?

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[jthomson]

I was toying with the idea of buying two bearings and replacing the other one when it goes. I found some on ebay form FAG. Does anyone have any recommendations? I guess it depends on the quote I get from the garage.

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[jthomson]

Uh oh... Put the car up on the hoist today and found the noise coming from the diff. Local Audi/Porsche guy doesn't work on diffs because the tools are too expensive. So... gotta truck it to Mpls. Sounds like it could be one of the two taper roller bearings on the left side. My ? is, what work do I do in there while it's open?

And yes, I'm prepared to take out a loan...

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