Todd Gibbs

Morning Gents!

Looking for some fresh ideas to help me track down an alarming noise. Let me describe it...

When driving (at any speed) there is a CLUNK Clunk clunk noise over bumps.
It is relatively loud and easily heard over the engine/road noise.
It seems to originate from the driverside (UK car) rear corner.
It can be felt through the chassis (ie, touching the handbrake cable tube or the body beneath the rear seats I can feel the knock knock knocking)
Bouncing the back of the car up/down on the suspension yeilds no noises.
I have checked extensively beneath the car and nothing appears loose.
It happens regardless of the gearbox being in D, 2, 1 or N.
The wheels are all bolted on tight
The Callipers are all bolted on tight
The hub has no play or wobble
All joints on that rear corner appears to be tight and without exccessive play
It started around 2 months ago (I rarely drive the car, but it was not present a while back)
It is present all times, from the very moment I start driving until I stop (ie, it doesn't need warming up)
I have taken calliper, disc and hub off and all seems ok, including the handbrake shoes.
It stops if I press the brake pedal <-- hopefully this is key... and why I am worried!

Really appreciate some guidance on this - Ive looked at everything multiple times and cannot see what could cause it, but its so loud and obvious it seems like it should be easy to find. If its useful I can provide a quick video.

Thanks!
Todd.

Mrmerlin

Check the rear axle nut 320 ft lbs also look at the sway bar links and the upper and lower control arm bolts the lower eccentric bolt will make noise if it’s loose 140 ft lbs
check the CV joint bolts 48 ft lbs
Note what was the last thing you fixed

Todd Gibbs

Thanks MrMerlin - I will recheck all of these, but I believe they are all tight. Last things I did : create new exhaust (but it happened just before this), rebuild all callipers (but it happened a long time after this), refreshed fuel tank and all pipes (long ago).

I will post a short video. It really sounds like, for example, the fuel accumulator loose, banging against side of body.

Thanks!

FredR

Todd,

Did you inspect the CV joints during your rebuild? If the shaft is solidly bolted then one of the CV joints on the offending side would be the most obvious suspect trying to find the cause of such a noise assuming you hear this every time the car starts rolling.

Knowing a little about your project I would think you probably replaced the boots and regreased the joint while you were in there. Is it possible you unknowingly lost a ball bearing from the joint?

Todd Gibbs

Video link
I did just find a loose exhaust heat shield, although Im sure the noise was there before i rebuilt the exhaust... will test again today

Thanks all.

Todd Gibbs

hi Fred - I have done very little to the rear, literally just rebuild the callipers and replace brake lines.

The drive shafts make some noise if I move them in/out, both left and right are the same. Seems fairly expected. Rotating the wheel and I get a small click noise from gearbox and cv joints, but nothing major.

The noise is only over bumps, otherwise it’s quiet. I just posted a video, and found a loose heat shield, but I doubt that’s the issue as it stops when I lightly press the brake

thanks!

FredR

I get the impression from the video image that the car is bouncing around in synch with the noise being made.

If correct that is not going to be caused by something minor like a heat shield being loose or whatever and suggests to me something is wrong in the drive train that is seemingly bouncing around rather chronically when it should not be.

Suggest you do further checks before driving around like that and risk serious mechanical damage or so I would be concerned about.

FredR

Todd,

If you have not touched the half shafts then maybe you should have. My half shafts were perfect when I replaced the boots a few years ago. Are you sure the grease inside the boot has not dried out? The CV joint as its name suggests [constant velocity] should have no radial play whatsoever and should be able to move freely in the axial plain - its very "raison D'etre".

Todd Gibbs

Hi Fred, no, I didn't touch the half shafts. There is no radial play, or minimal, and I can move them freely in/out in the axial plane.

Rewatching my video (sorry the 'rattle' text is badly placed) the camera is bouncing up and down, which is making it look worse than it is. Its just the sound that is relevant really.

Thanks for the help
Todd.

Todd Gibbs

Hi MrMerlin,

I re-checked all the front suspension components, all tight. The sound really sounds like its from the rear, and stops if I lightly press the brake pedal. I was expecting to find the calliper loose, but its tight.

As far as I understand, these are floating callipers? could that cause this type of sound?

thanks again
Todd

Mrmerlin

For what it’s worth if your body is bouncing around then you need new shocks. Otherwise good shocks the body on a 928 is it gonna move much if at all especially trying to push it by hand.

FredR

Todd,

My effort with the half shafts suggest that one cannot really tell the condition of the CV joint whilst it is in situ but that may reflect the limits of my capabilities. On the other hand when you have the shaft in your hands it is very easy to feel whether there is any radial play. To get them out [if you feel such is warranted] you need a 3/4 inch socket set and a big lever arm- that or a very powerful impact gun.

Replacing the boots and cleaning out/changing the grease is a once in every "10 year job" as I am concerned and given your 928's background or what I understand of it such action would seem prudent no matter the actual cause of your perceived issue.

Rgds

Fred

Mrmerlin

from that noise it sounds like a exhaust system is hitting a hard part

Rob Edwards

Speed-dependent knocking like that that goes away with the brake pedal sounds to me like a bad CV joint that's knocking when loaded in one direction and then the joint gets loaded in the opposite direction (and stops knocking) under braking. Or a bad wheel bearing (? Though that should vary with turning left or tight/varying axial load..). Are the CV boots intact? And the CV joint bolts are all tight to at least 60 ft-lbs to the transmission output shafts?

h2pmr

hi Todd
noises and vibrations etc can be a real pain to find, i try to reproduce things like this when the vehicle is stationary, need a 4 post lift to keep the vehicle on all 4 wheels, then get someone else to bounce the body up and down while the vehicle is up on the lift and you are underneath, if you still cannot hear anything try holding components, shock absorbers etc to see if you can "feel" the noise with your hand.
could also try getting someone else to drive it on a private road while you sit in the tailgate area with the tailgate carpet removed to see if you can pinpoint the noise.
if it does it at slow speeds open the tailgate while its being driven, the more you can pinpoint its location, the number of things it could be decreases.
good luck
Phil