996 Wheel Bearing Replacement

Hi Team, I started to notice a feint humming noise earlier this week and so have been trying to diagnose. Its around 56-60mph and seems fairly rhythmic whirr/hum. If I put the car out of gear or dip the clutch at that speed the noise is the same, and if I cruise at that speed and switch from light accelerator application and back to coasting, it still seems to be the same noise although obviously it appears to be more prominent when coasting then when driving the wheels due to the increased engine noise. I hope I've eliminated the gearbox/diff pinion and bearing as likely culprits so am thinking wheel bearing. After a several runs in different conditions I've taken the temperature of the aluminium wheel spacers with a laser thermometer, despite other variables, the nearside rear has always been 7-10 degrees C warmer than all the others. Some variance between front and back wheels which I've attributed to braking differences, but always nearside rear is warmer. Does this sound conclusive that the bearding is dying? Next question, is whether to get it done at my Indie as it's going for service next week (to get a due stamp this time), or to do it myself. I am weighing up buying a 20T press for about £120 and an FAG bearing kit for about £60 versus whatever the indy would charge. Logic being I have 3 more original bearings on this car, and 4 original bearings on my X5 so getting "tooled up" might make more sense than taking the easy route and having it done. Anyone done the job before? I am a bit worried about removing the carrier. I had to cut out and replace my rear toe arms and replace the eccentrics, and replaced the disks recently, but slightly concerned about fatal galvanic corrosion on both dog bones or the handbrake mechanism exploding in an array of random springs I can't put back together! Any tips?

That does sound more wheel-speed related than engine/gearbox, especially if dipping the clutch or coasting makes no real difference. The rear wheel bearing would be high on the list, but I wouldn’t call the temperature difference conclusive on its own. A slightly dragging brake, handbrake shoe or calliper (or is that caliper) could also make that corner warmer. I’d check that corner for play, roughness when rotated, brake drag and also inspect the tyre for stepping, as tyres can mimic bearing hum quite well. On the DIY side, the bearing/press work is manageable. The bigger risk is seized 996 rear suspension hardware and the handbrake bits turning it into a longer job. As it’s going to your indie next week, I’d probably let them confirm the diagnosis first, then decide whether you want the battle.

One other way to diagnose a bearing is to see if it goes under load , so let's say you suspect it's the nearside front , when you go around a right hand corner you're loading up that bearing an the noise should go momentarily , it should also be more noticeable when you go around a left hand bend taking weight of it and it making more of a fuss. Edit...got that round the wrong way Pressing out and pressing in bearings is relatively straight forward but you do need the correct size tools to make sure it goes really smoothly. Might be front (i can't remember ) aluminium disc shield that often corrodes and might need replacing , so a good time to do that whilst the hub is out, but remember to put it on though before you press the new bearing in!!! I forgot!

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Thanks, yes I think asking the indie to help diagnose makes perfect sense. If they can run it up on the ramp and listen with a stethoscope it should be able to at least confirm it's not a gearbox issue which would be my primary concern. I've done a full front suspension refresh on the 996 and complete replacement of everything on my E53 X5 so am used to wrestling with suspension, and that is OK when my intention is to wrestle/replace suspension arms/bushings etc... On this occasion I am hoping to go in, change the bearing and put it all back together again without the extras... if I have to cut out and replace dog bones, I may as well do both sides, which means both bearings... then there's the risk of damaging the coffin arm ball joint gaiters... and so on.... as we all know! But if I buy new forks, coffins, handbrake shoes, dog bones and bolts etc in advance, sods law the original ones will all be fine when I remove them!

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Yes thanks. forgot about the shields, always worth changing them when the hub is out. I tried the "F1 warm up weave" to see if I could hear any changes due to load, but I couldn't really tell and probably looked like a berk doing it too LOL

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It should change noise levels when you weave especially the front , not so much the rear. Have you raised each wheel and span it and tried top and bottom wiggle? One other thing , I'd leave the ABS sensor alone if it's working, they don't like to come out.

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I am suspecting the rear and it was very difficult to discern any difference whilst changing direction and still watching where I was going, and of course it's quite a noisy car anyway. I've not had the wheels off the ground yet but may take a look at weekend, it passed MOT with no advisories less than 3 weeks ago (no noise then), so if it is a bearing it's probably in the early stages as I am sure my MOT tester would have noted any 12-6 play. Yes disconnecting the ABS sensor at the wire and leave the sensor in the hub sounds like good advice. I've just found a similar thread where Alex said the cover plate bolts all sheared and needed machining/re-tapping so as much as I prefer to overcome these challenges myself, I may need to plan for more than a few hours on a Saturday morning... (hope for the best, but expect issues!)

Yeah I've not had any issues with any of the cover plates but I remember Alex having "problems". Good luck and let us know what the diagnosis is.

The workshop manual procedure is to heat the casting to approx. 100°C but not over 120°C (so as not to compromise heat treatments to the casting in production) before pressing in the bearing. It is not a simple press fit.

And put the bearing in the freezer for a few hours.

Wheel bearings can be tricky to diagnose on driven wheels, I’ve found in the past Ive removed caliper / drive shaft to eliminate any contributing factors, you get a much better feel for it then, Time consuming admittedly The load up trick while driving Monty mentioned is usually a really good give away. I haven’t done 996 ‘yet’ so leave technical to others till I have. You got a friendly local garage could use their press ? Maybe indie would do that bit save you having a press kicking about in your way at home .. I don’t know your set up but you seem very aware what you’re in for, if you fancy scratching about red faced on the floor fill ya boots ! Keep us posted

I've been on the hunt for a really nice old press from an old school workshop , nothing to big , benchtop for pressing bearing in and out. There are loads of crappy things for sale but of new stuff I think the Clarke ones are OK. I currently use my 500 megaton log splitter and therefore have to be extremely careful as it'd squash the entire hub very easily.

This looks like a good one.. https://www.ebay.co.uk/itm/117136954993?mkcid=1&mkrid=710-53481-19255-0&siteid=0&campid=&customid=&toolid=10001&mkevt=1

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Ever so slightly. Pressing the bearing in and out was a doddle. Removing the bolts didn't happen. I had to drill each one out and redo the holes......on both sides. That was the front, don't know what the back will be like. PS, you don't need to waste money on a press to get the bearings in and out.

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Not actually good practice as the condensation that forms on it when you take it out can contaminate the lubricant or form a potentially corroding environment as it will be going into an alloy housing.