Droning noise from the rear - 97 3.2 Carrera

[smithH]

Hi all,

I have a loud droning noise from the rear of the car. I have heard this could be rear wheel bearings or a diff bearing. Can anyone suggest how I might tell which it is?

The noise doesnt get worse around corners, and is quite rythmic at about 80mph in 5th its really quite noisy!

I have read the archives on rear wheel bearings but am still not sure. Can I do something like take car for a drive and see what bits are hot? What should I be looking for.

Many thanks

H

[Lucky]

Please share # of miles and any work done in the past on the rear suspension/tranny/half-shafts/etc.

~~~~~

I had similar concerns a couple of years ago when my car quickly developed a loud rumbling from the rear. I was convinced the rear bearings were going. Turns out it was tires. Tires age from just sitting around, so even though the bad ones on my car had less than 10k miles they had turned to rock from age!

What kind of tires? How old? How many miles on them? Do you do DEs (too many heat cycles kills tires too)?

[smithH]

Sorry a bit of the history.

104,000 miles, new tyres about 3,000 miles ago. New CV joint about 5000 miles ago. No other rear end work. I do track days, but not for about 6 months.

Could be tyres, I guess you can only tell by swapping the wheels. the noise is getting a bit worse too.

[avi8torny]

wheel bearings if it's not the tires

[Superman]

Probably a wheel bearing. Raise one side of the car, and be sure to block the wheels on the other side so the car cannot move. Start the car, put it in gear and get the wheel moving, then depress the clutch and shut off the engine. Repeat for the other side. You'll hear clearly.

[smithH]

Let me make sure I have got this right, you rely on the diff to turn the one wheel and not to drive the wheel left on the groung?

I'll give this a go and report back.

[Superman]

H, before you do something silly like follow my advice, make sure you have the right kind of rear end. My car has a plain ol' transaxle final drive, so the procedure I described works just fine. Hopefully the car you are dealing with has the same rear.

[Lucky]

If you have a limited slip rear-end, then Superman's idea won't be so super. Also if it is a bearing, would it make much noise without any weight on it?

[Superman]

Well, I hope he has not made the mistake of listening to me. As far as weight goes, yes you can usually hear a bad bearing even when there is little or no load.

[Steve W]

Yes, bad rear tires can definitely sound exactly like bad bearings on a 911 - speaking from experience.

[masraum]

If you have a limited slip rear-end, then Superman's idea won't be so super

Thanks, I needed that.

[Superman]

Well, I'm hoping smithH did not use my advice to create a Porsche-sized hope in his garage door. I'll feel better when he returns.

[smithH]

Relax guys, I havent done anything silly. The car is a standard sport coupe which I am 99% sure didnt come with a LSD. I havent tried your trick yet, but did take the car for a drive last night. The noise is linked to road speed not engine revs and is still there when coasting with the clutch in.

Unfortunately this means I am no further forward on whether it is bearings or differential noise. I am not sure I have "room" in garage to do the one side up trick. I would feel happier knowning I had a bit more space. I think I will have to look at taking it to my Porsche garage and get them to take a look.

Thanks for the help - will let you know how I get on.

[Superman]

Whew!

I have had little experience with bad rear end gears (differential, transaxle, whatever), and it seems that at least nine times out of ten the problem is a wheel bearing. Rear wheel bearings can be a little dicey for the home repair dog, so I'll not disagree with your plan to take the car to a trusted pro. Still, it might be illuminating to carefully spin each of those rear wheels separately. I think you will find one side noisier than the other.

Rear ends usually do not go bad, as long as they were assembled to spec.

[smithH]

Superman thanks for the advice, I'll give your trick a go if I feel brave enough.

Glad you agreed on doing the bearings at the garage. Didnt fancy the job much, having read the archives, but hate it when the car gets the better of me.

[Superman]

I always do these things, even when I shouldn't, but that's because I'm not very smart. For example, not getting the rear axle nut properly tight can ruin the bearings ($80 per), the splined stub axle and the other splined part (Who knows how much $ this mistake would cost), in addition to possibly messing up the aluminum trailing arm (not cheap....not cheap at all).

[beepbeep]

Originally posted by Superman
I always do these things, even when I shouldn't, but that's because I'm not very smart. For example, not getting the rear axle nut properly tight can ruin the bearings ($80 per), the splined stub axle and the other splined part (Who knows how much $ this mistake would cost), in addition to possibly messing up the aluminum trailing arm (not cheap....not cheap at all).

Been there, done that!

It cost me around 2000$ to fix it. Had to change banana-arm, bearings (there are two on 930), handbrake cable etc.

That being said, it's quite easy to diagnose bad bearing. Just lift up one side of the car with OEM jack and turn the wheel by hand. You'll notice that wheel with bad bearing isn't turning smoothly. Carreras and SC's are much cheaper to fix as there is only one bearing. Probably DIY-job...

Mine sounds like this at constant motorway speed in fifth:

rrrrrRRRRRRRRrrrrrrrRRRRRRRRrrrrrrrrrRRRRRRRRrrrrr

Kind of like an old refrigerator.

My guess is it's the gearbox/differential. Wheel bearing sound wouldn't come from the middle of the rear and furthermore it would not just be noisy at constant motorway speeds.

[Superman]

Mikkel, I wouldn't make those assumptions. Possibly your growl is not a wheel bearing, but maybe it is. Wheel bearing noise can be deceptive.

[Eric Coffey]

Yep, sounds like a bad bearing, and the rears aren't too DIY friendly. However, you shouldn't rule out a bad CV either (especially if you have a torn boot). Good luck!