Unusual Noise Has Developed In My 911

[dkirk]

I’ve recently experienced an unusual noise that’s developed in my 1988, 911 coupe. Best described as a low-frequency “humming” noise, most apparent at 60 mph and above, and associated vibration can be felt in the steering wheel. The hum changes frequency and is proportional to vehicle speed. At speed, disengaging the drive train (by pushing in the clutch and engaging neutral) has no effect on the noise. There is no apparent vibration felt in the shift lever so I don’t suspect any issues in the drive train or rear end components. At speed when turning left, volume increases; when turning right, volume decreases.

After returning from a previous drive when the noise was most pronounced, I grabbed the top of the RH tire and felt some excessive axial play in the wheel bearings. The LH side was to spec regarding end play. Thinking I found the problem area, I ordered new inner and outer bearings from our host. Getting the car up on the rack, the RH end play was evident. Removing the grease cap showed ample grease in the bearings which were originally repacked at 100k miles (car currently has 159,600 miles). The clamping allen bolt was tight in the wheel bearing lock nut. Removing the bearing hub and bearings showed that the grease in the inner bearing was turning black. Seal and sealing surface looked good. Outer bearing races were pressed out and bearings examined after cleaning. No distress was noted on the outer races or on any of the rollers. Bearings rotated smoothly by hand. I did notice an unusual finish on the thrust surface of the inside, inner bearing race, shown below:



Looks as if the inner race was rotating against the shoulder of the spacer (referring to Bently illustration page 401-11). Does this appear typical on the inner bearing?

Old races were driven out of the hub, new races pressed in, and hub and new bearings were assembled and packed with new grease. Wheel bearing lock nut was adjusted to instructions given in the Bently manual and the clamping allen bolt torqued to 15 Nm. Hub rotated freely with no discernable axial play noted. The following day, car was taken on a 100-mile drive and the noise was totally gone as well as vibration felt in the steering wheel. I thought problem had been resolved.

Next time out (approximately 48 hours later), the humming noise and vibration had returned! Jacked up front of car and noted some axial clearance had developed in the LH wheel. Removed grease cup and found that I could get another ¾ turn on the wheel bearing lock nut to get proper clearance back. Incidentally, the clamping allen bolt was still tight. This time, I increased torque on this bolt just to be sure the wheel bearing nut was not moving. Reassembled, test drove the car, and the noise is still there – no change!

I should note that I’m running Continental Extreme Contact Sport tires which are only 1-year old and have accrued 4900 miles since installation. Tire pressure is the recommended 29 psi front, 36 psi rear. No visual defects noted on any tire. All tires are in balance. No defects noted on the original Fuchs wheels. All lug nuts torqued to the 96 ft-lbf spec.

I hope I haven’t been too detailed in this explanation but this problem really has me perplexed. I would appreciate any advice or suggestions from those experiencing a similar problem with their 911. Thanks in advance.

[john walker's workshop]

Usually it's the outer bearing that fails. The inner race pits. Evident by little metal flakes in the grease between the rollers. They're dirt cheap, same as a 70s vw bus. Clip the cage off and inspect the inner race to see, then get a new one.

[ant7]

Are you sure its not one of the rear wheel bearings ? I had a similar issue, which made me think it was the front nearside wheel, after changing that bearing the noise was still there, long story short, it turned out to be a rear wheel bearing, after changing both sides on the rear, all was good.
Ant.

[dkirk]

Thanks for the suggestions guys. Another owner suggested replacing the front LH bearing, just to be consistent. I have the parts and the job is relatively easy so I'll go this route and report back.

[Magyar Kiwi]

Hi, some comments
Front wheels
As you have noticed the inner races are free to turn on the stub axle.
Sometimes, (particularly at the smaller diameter outer location) this leads to wear on the lower surface of the stub axle. If this wear is excessive it causes excessive free play at the wheel, which cannot be removed by adjusting the clamp nut.
Stub axle wear can be repaired by plating or replacement.
Rear wheels
Rear wheel bearing noises often come and go with gentle left and right steering inputs which alternately load and unload the noisy wheel bearing.
Enjoy.

[dkirk]

This weekend, time permitted to investigate the left side wheel bearing and obviously, the outer bearing was in the process of failing. Severe spalling was obvious on one of the rollers and impact damage on the outer race from rollers running over debris. The inner bearing and seal appeared fine. Both bearings replaced, packed with grease and all is now well. No more noise and no more vibrational feedback into the steering wheel.

I thank all of you that contributed the advice and suggestions.