DDK - Die Deutschen Klassiker

As ever, my 987.2 Boxster sends me challenges just when I've fixed the last one.
The current issue is a bit odd, but some one here may have some experiences that may help me track he problem down.
This could easily apply to any car maybe.
Recent eurotrip and a humming noise could develope at around thirty mph and be drowned out by road noise at sixty ish.

I chose to ignore it and got the coil pack failure fixed.

We are in Scotland now and the humm is noticeable on smooth roads, same speeds.
Engine or gears selected makes no difference, humm is just louder than the engine but irritatingly noticeable.

Possible sources
Tyres ar rear, but half worn and no damage anywhere on the outside.
Wheel bearing, no play can be felt, out of gear and no handbrake on.
Driveshaft cv, no low speed clicking on tight turns etc.

At fourth, swerving left to right etc and the humm does not change at all, it only changes loudness with speed.

Any thoughts please?

roof up or down change it ? Could it be something effected by airflow , eg has someting at the front or rear come slightly loose and is deflecting in the air and vibrating

I recently had this issue with my daily, thought it was the rear wheel bearings but turned out to be feathering on the inside of both rear tyres due to a bit of age related sag in the suspension.

Had the geometry tweaked and fitted a new set of rears and noise has disappeared.

Definately worth a quick check I'd say

Thanks!
Before the eurotrip I had the wheels off the car to check the suspension, clean things down etc for the MoT.
I am careful to replace each wheel on the corner it came off, maybe I got the rears wrong.

The tread on all 4 were impressively true, nice and even etc and no pot hole damage.

The humm showed itself on the trip, long runs on smooth tarmac, but not too bad.

Now it is louder, but on a long road run in Scotland on the super smooth A9 it is certainly there, 30 through to 60 when road noise in general overtakes the humm.
To 30 ish, all is quiet.

I would expect wheel bearing issues to be audible at 30 and in fact at all speeds.

Plan is to jack the car up and spin each rear wheel by hand. No weight on the wheel, no brakes and see how that feels and sounds.
I am expecting silence.
Similarly, the drive shafts, check cv boots are good etc and the shafts float axially.
I'm expecting nothing to report.

I can check any plastic covers if loose.

I'll swap the rear wheels over, drop the car and drive it to see if anything has changed.

If this results in the humm still being there I'll take to Zuffenhaus who service the car for an hour with them on the ramps to see if they spot anything.
They can also check the rear alignment.

That will leave the tyres...

It could always be the tyres themselves. Some can develop some harshness the lower the tread gets, due to the compound getting harder. I had Bridgestones that made the car sound like a wheel bearing had failed when they got low - changed the tyres and noise disappeared.

These are 19" N spect Bridgestones all round...1/2 worn....
Quiet as a door mouse when new.
Fronts are also Bridgestones and super low profile.

I had this on my daily driver not too long ago.
I was convinced it was a wheel bearing but was in fact feathering / low tread on a pair of my tyres.

Changed the tyres and all the noise went away.

My observation was that if you went over an undulation the hum would get louder whilst the suspension was in compression and disappear briefly as the suspension unloaded.

Regards
Andrew

911hillclimber wrote: ↑Wed Oct 04, 2023 6:08 pm These are 19" N spect Bridgestones all round...1/2 worn....
Quiet as a door mouse when new.
Fronts are also Bridgestones and super low profile.

Funnily enough mine were Bridgestones as well!

It’s usually the simple things .. but engineer heads always assume the worst

As if...
But true.
Light bulb goes out, change the power station first, then the bulb.
Hope it will be the tyres.

Day after getting back from the Scottish trip, time to see if anything obviously wrong at the back of this car.
To be checked
Tyre shape/damage/wear
Wheel bearing noise/feel etc
Drive shaft play inc the ease to plunge in the CV's. Boot damage etc
Any loose wheel arch covers etc, loose pipes or otherwise (check brake wear too)
Reverse the wheel positions.
Go for a 50/60 mph drive and see if the humm has gone or changed.

Results in order from above
One tyre not as round as the other using a wax crayon marker against the tyre face when rotating wheels. Ovality about 1mm or less. (correct pressures, 34 psi)
No tread wear, tread about 2mm from raised wear rib, nothing uneven, no inner thread wear, both looked good.

Absolutely zero bearing wear/movement/ silence/play, both solid as granite.

No play, just usual slack when in gear, play from gearbox. Shaft plunges beautifully on both sides, smooth as can be, all gaiters sound.

Nothing loose in the arches, suspension etc, all solid.

Wheels WERE on the opposite side to the positions when new. My mistake early in the summer. Now wheels on correct sides.

Test drive. Noise might have gone, so difficult to be convinced, but different in a good way to the long runs in the car over the last few days in Scotland.
Will use the car this week and take the wife too, she has the hearing of an owl.

All worth doing even if not 100% conclusive.