Front end thrum, thrum, thrum, thrum noise...
Discussion
As title. Increases with speed, most clearly heard at around 20-25mph, just degenerates into noise as the speed increases.
Wheel bearing?
I did some rapid calculations.
Wheel circumference 200cm
30kmh = 8m/s = 4 tyre revolutions.
I was convinced that the frequency of the "thrum" was 2Hz, i.e. 2 tyre revolutions, but realise that the result indicates 4Hz
I may have timed the frequency wrong (did not actually time it, just guessed it, but perhaps my second was actually half a second i.e. a second is longer than I imagined it to be)
Or perhaps it's not a wheel bearing. Something diff related.
Yes perhaps hopefully not imagining things but got the impression that loading the nearside very very slightly increased the volume, then again going round a corner would also involve the diff...
Anyway timing myself just now as I post, counting numbers against a clock at the approx frequency heard, seems to give an implausible 2Hz (too slow) and a more likely 4Hz, i.e. I could count to 20 in 5 secs but counting to just 10 in 5 secs it is clear that the "thrum" was faster.
Anyone BTDT?
Wheel bearing?
I did some rapid calculations.
Wheel circumference 200cm
30kmh = 8m/s = 4 tyre revolutions.
I was convinced that the frequency of the "thrum" was 2Hz, i.e. 2 tyre revolutions, but realise that the result indicates 4Hz
I may have timed the frequency wrong (did not actually time it, just guessed it, but perhaps my second was actually half a second i.e. a second is longer than I imagined it to be)
Or perhaps it's not a wheel bearing. Something diff related.
Yes perhaps hopefully not imagining things but got the impression that loading the nearside very very slightly increased the volume, then again going round a corner would also involve the diff...
Anyway timing myself just now as I post, counting numbers against a clock at the approx frequency heard, seems to give an implausible 2Hz (too slow) and a more likely 4Hz, i.e. I could count to 20 in 5 secs but counting to just 10 in 5 secs it is clear that the "thrum" was faster.
Anyone BTDT?
Might be too obvious but.....
I had an issue with the brakes on my old 968 where a small strip of rust would build up on the disc under the pads, if the car was left standing for a while. Whilst driving this produced an odd noise similar to that you describe. Apparently common problem cured by some vigorous braking!
I had an issue with the brakes on my old 968 where a small strip of rust would build up on the disc under the pads, if the car was left standing for a while. Whilst driving this produced an odd noise similar to that you describe. Apparently common problem cured by some vigorous braking!
More info - and thanks to all who contributed above -
It is definitely a "bearingey" noise, brakes are fine. No vibration under braking either.
Yes 996TT front diffs may have issues, hopefully not in my case...
Tyres, no, recently replaced, barely worn, and as above, bearingey type noise.
One thing I am remembering just now - not all that many miles ago, probably less than 1.5k, I was driving rather slowly and heard a nasty sound coming from what appeared to be the direction of the nearside front. A loud crack - scrunch type sound. So nasty that I immediately got out of the car and investigated. I could see nothing wrong. Did not appear to be anything trapped between disc and cover plate, nor in the sus, could see nothing. Noise happened again within a few hundred yards, but since then, absolutely nothing.
Perhaps this was the beginning of the end for the bearing? Hopefully, the WHEEL bearing?
It is definitely a "bearingey" noise, brakes are fine. No vibration under braking either.
Yes 996TT front diffs may have issues, hopefully not in my case...
Tyres, no, recently replaced, barely worn, and as above, bearingey type noise.
One thing I am remembering just now - not all that many miles ago, probably less than 1.5k, I was driving rather slowly and heard a nasty sound coming from what appeared to be the direction of the nearside front. A loud crack - scrunch type sound. So nasty that I immediately got out of the car and investigated. I could see nothing wrong. Did not appear to be anything trapped between disc and cover plate, nor in the sus, could see nothing. Noise happened again within a few hundred yards, but since then, absolutely nothing.
Perhaps this was the beginning of the end for the bearing? Hopefully, the WHEEL bearing?
I had a similar experience a couple of weeks ago. Short screeching noise followed by an on-going thrum, thrum, thrum...
My first thought was wheel bearing, but I got the wheel off the next day and had a good look around. I was about to give up and book it into the workshop when I found a large stone had somehow wedged itself down into the brake caliper on the inside of the disk. Some long-nosed pliers got it sorted.
My first thought was wheel bearing, but I got the wheel off the next day and had a good look around. I was about to give up and book it into the workshop when I found a large stone had somehow wedged itself down into the brake caliper on the inside of the disk. Some long-nosed pliers got it sorted.
Jacked up and tried rocking wheels, no play evident. But I have read up on this on rennlist etc and the consensus is that there is unlikely to be any evident play unless the bearing is really, really shot.
And, I saw a pic of a bearing race that was damaged in just one quadrant which would explain the thrumming noise.
And furthermore, that wheelbearing failure is quite common, even at low mileage (20k) so mine at 67k is well overdue.
Car has always been noisy, assumed that's the way it is, but perhaps the bearing/s have always been dodgy.
I will replace them and hope it gets sorted.
And dammit just noticed (well maybe the "bearing" issue has a silver lining to it...) that one CV boot is torn and I have fresh grease spattered inside the front rim. So caught in time, and will prevent damage to the joint itself.
And, I saw a pic of a bearing race that was damaged in just one quadrant which would explain the thrumming noise.
And furthermore, that wheelbearing failure is quite common, even at low mileage (20k) so mine at 67k is well overdue.
Car has always been noisy, assumed that's the way it is, but perhaps the bearing/s have always been dodgy.
I will replace them and hope it gets sorted.
And dammit just noticed (well maybe the "bearing" issue has a silver lining to it...) that one CV boot is torn and I have fresh grease spattered inside the front rim. So caught in time, and will prevent damage to the joint itself.
. But would we give them up for something "sensible" ? Not a chance 
.supermono said:
Wasn't there an issue with the front diff or something on these cars occasionally. I'm sure I read about it once, maybe that?
(Assuming a 996TT02 lol)
He's right you know. Get thee to a garage, nice and slowly... Why?(Assuming a 996TT02 lol)
Because I have just spent £3k on a replacement final drive (front diff..) having experienced exactly the same noise at low speed.
The likely culprit, possibly a bearing failure, but..this will also mean the seals have probably gone, which means you're probably leaking fluid, which means your front diff isn't lubricated....so it's chewing away in there....which is where the money is...
Mine (front diff) was repaired only at cost as the OPC accepted they have overlooked something before returning my car; I won't say which OPC as we have an agreement. You cannot repair these units, they are sealed (except when they are not through a failure..) you need a replacement part - but only if there is carnage going on within the diff and a magnetic plug will tell you 'how bad things have got'; mine looked like a hedgehog and there was 3/4" of play around the bearing and seals - so no seals then...
Let us know how you get on, but get it off the road before it costs you 5 sets of tyres..
Edit: Apologies, having just read that it is pretty spurious! I have a C4S, it has the same diff as your Turbo, my bearings failed, as did the seals on the diff, the lubricant leaked out, the OPC didn't pick it up (but picked up lots of other things to charge me for, even though it went in for the "thrum, thrum." you mention). I drove the car, the thrum remained, the diff got chewed up. "I was annoyed they missed the problem", they agreed that service missed something somewhere, replaced the unit at cost - no labour and unit cost from Porsche not retail. Car running fine, except that on the way back from Silverstone today one of the cylinders is misfiring - FFS......am off to the Indie! Have owned my 996 for 5 years.
Edited by RoadRaider on Saturday 4th July 17:46
Edited by RoadRaider on Saturday 4th July 17:48
Edited by RoadRaider on Saturday 4th July 17:49
Update, replaced both front bearings, one was definitely well shot other still OK. Result, noise gone... silence... now perhaps I am making out the rears... or maybe I have been hypersensitised and just imagining things!
Anyway when car was jacked up noted a leak from aircon refrigerant pipe coupling... damn... replaced o-rings but hope that's it, we'll know pretty soon enough.
Anyway when car was jacked up noted a leak from aircon refrigerant pipe coupling... damn... replaced o-rings but hope that's it, we'll know pretty soon enough.
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