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DIY: Do It Yourself
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Old 04-01-2012, 11:41 AM   #1
Stanleyf233
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Thumbs down Strange Knocking under Acceleration

Now my problem is this. When I accelerate the car there is a knocking sound that appears to be occurring somewhere around the rear left of the car. The knocking gets faster as the wheels picks up speed and seems to be cyclic in nature. I thought this might be a RTAB, so whipped of the wheel and had a look and to my surprise found a small piece of RTAB. Thinking I had found the culprit I ordered up my Powerflex replacements. On removal of the LH one I almost convinced myself this was the defect due to the state of it. The out case had almost corroded to nothing in parts so even if it didn't clear the knocking noise it has fixed something that was particularly broken.

The suspension has been changed to koni adjustables. The big bush on the diff has also been changed to a Meyle HD. Below are the pics of the RTAB bushes I pulled and subsequently replaced.

Now I know my RTAB's are good, Shocks and springs are good and big diff bush is good.

What else can it be?
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Last edited by Stanleyf233; 04-01-2012 at 01:11 PM. Reason: Forgot to mention
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Old 04-01-2012, 02:05 PM   #2
Crack_Fox
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Any boot floor cracks/metal fatigue?
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Old 04-01-2012, 03:01 PM   #3
Stanleyf233
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No nothing that i can, and no witness marks from metal to metal contact either. the only thing i did see is the LH Drive shaft seems to move into the diff more and easier than the RH one.
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Old 04-02-2012, 01:07 PM   #4
eeleaj
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Maybe check your center support. The rubber surrounding the center support bearing can be torn and the bearing can move around in the center support in a vertical direction up and down and you will feel vibration under load but normal during cruising.
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Old 04-02-2012, 04:04 PM   #5
Stanleyf233
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Starting to think its something in that area as well, so time to lift the car up again. but cheers for the input.
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Old 04-02-2012, 04:50 PM   #6
eeleaj
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No problem. I was chasing the knocking in the rear of my car until I found that torn center support. After replacement the knocking sound disappeared under hard acceleration
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Old 05-21-2012, 04:27 PM   #7
Stanleyf233
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The Knocking Saga continues.......

So the knocking continues on. I have done some more investigation and have still not found anything substantive. What I have found is that it is definitely somewhere between wheel (rear left) and the diff. Having recorded the noise using my phones voice recorder. Looking at the file as a waveform, it was possible to time the knocking and after working out the wheels RPM at certain speeds and starting the knocking from a known speed I can calculate how often the knock should occur and compare the two. If the knock had been approx 2.93 times faster, it would have meant it pre-diff. Now it is certain that it on something between the tyre edge and the propulsion point for the left output shaft.
Does anyone have any ideas? There is now only a finite amount of things it can be. It only happens in high torque conditions. So this means it is either the torque itself there by leading to the CV joints, splined shaft, attaching bolts or wheel bearing. Or it is a product of the torque (acceleration) causing the geometry change in the suspension causing something to touch, once per wheel revolution.
It does count out things like the guibo and CSB though. In my original post it may have been misleading when I said drive shaft as well, the correct (by the book term) is output shaft.
Any words of wisdom are greatly appreciated, I am personally heading towards the outer CV joint at the moment but don't really go in for the willy nilly part change. So all received information will be thrown into the mix and a video recording will be made so I can post as it won't accept the *.mp3.
Cheers in advance

Last edited by Stanleyf233; 05-26-2012 at 04:51 AM.
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Old 05-21-2012, 05:41 PM   #8
SamDoe1
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What about the brakes? This includes the E-Brake. Get the car up on a lift and have a buddy put the car in gear and accelerate. Then stand near the rear wheel in question and listen for where the noise is exactly coming from.

Wheel bearings don't typically, at least I've never heard it, make a knocking sound.

It would be nice to have a video or sound file. Preferably video.

Last edited by SamDoe1; 05-21-2012 at 05:42 PM.
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Old 05-22-2012, 01:22 PM   #9
Stanleyf233
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ok, here are files of the noise. cheers
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Old 05-23-2012, 03:12 PM   #10
Stanleyf233
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Try again

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Old 05-23-2012, 04:00 PM   #11
Stanleyf233
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this is the same noise recorded in the car.

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Old 07-23-2012, 06:12 PM   #12
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Ok, since my last post i have carried on looking, today I found this, it appears to be a crack in the spare tyre well on the left hand side of the car. I will be stripping out the boot tomorrow to investigate further.
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Old 07-31-2012, 05:53 AM   #13
ChrisE46330ci
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Hi Stanley,
I think I have the same problem, the knocking sounds identical only under slightly different conditions. Mine happens whenever the left rear is under load, not only under acceleration but also when turning right.

I have found that it doesn't only happen whilst there is power through the drivetrain but just turning right at a reasonable speed with the clutch in.

I'm now starting to think it could be the outer cv joint of the left rear drive shaft as I'm assuming the angle of the joint will change under load regardless of weather or not any torque is being applied. What do you think?

Hope i'm not hyjacking your thread only trying to open up discussion of possible causes.

Will try to get a recording today to confirm same noise and have a look under the car to see the cv boot is in good nick. I'll also check the spare tyre well for any cracks.
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Old 07-31-2012, 07:33 AM   #14
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Here is a recording, this was accelerating in 1st.

Do you think this is the same problem?
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Old 07-31-2012, 11:10 AM   #15
Stanleyf233
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Don't worry mate, your not hijacking anything i just want to solve this as it is getting proper on my nerves, now i just listened to your recording and that sounds similar i thought it WAS mine at first! i have also just spent a year in Scotland (2011) at Rosyth and i thought the roads around there after all that snow at the end of 2010 might have been to blame in ruining a bush. i have um and ar'ed about the CV joint (outer) a few times and i do keep coming back to it. have you checked for any cracks. if you could it would be helpful in eliminating the crack i have that can be seen in the pics i posted the other day. I can say it is definitely not RTAB's
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Old 08-01-2012, 02:47 AM   #16
ChrisE46330ci
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I replaced my rtab in January so yeah definitely agree there. I had a look for cracks but can't find anything. Also, I'm not sure how a crack in the subframe would lead to the noise we're hearing, not ruling it out but think it's unlikely. My cv boots look ok with no obvious signs of damage, but I really can't think what else it could be.
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Old 08-01-2012, 06:07 AM   #17
Stanleyf233
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Agreed, CV boots look fine, brakes are fine hand brake is fine. rear upper shockmounts are Meyle hd and changed last week, big diff changed last year to meyle hd andf is fine, RATB changed to powerflex at easter. I took timings of the knocking noise to isolate position by working out wheel RPM at certain speeds. comparing knocking frequency with wheel speed equaled one knock per rev. if it had been 2.93 knocks per wheel rev it would be propshaft side to diff input. but by calculation it means it must be diff output side to wheel. i have also placed a webcam under the car and filmed the hub to no avail. i am almost at the point where there can only be a few thing it can be:-

1. diff
2. CV joint
3. either of the ball joint or bushing on the hub
4. wheel bearing (though it would be a strange defect mode)
5. sway bar allowing to much geometry change
6. weak diff bushing (fwd)
7. Bushings on either of the arms.

The investigation continues.....
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Old 08-01-2012, 01:13 PM   #18
ChrisE46330ci
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I've done a little more research and found this page describing symptoms of a worn cv joint.
http://www.samarins.com/glossary/cv_joint.html
After what you've added and this I'm pretty convinced that's what's wrong. I'm away with work for a few weeks but will look into what needs replaced, either the joint or entire shaft, and plan to do the work when I get back. I'll keep looking online for other possibilities and keep you updated.
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Old 08-01-2012, 04:13 PM   #19
Stanleyf233
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I must admit, my very first thought about this was CV joint and have already looked into it. I don't think they are supposed to be replaceable. That doesnt mean that they aren't, but it may not be financially the best thing to do, I am 95% convinced it is the outer CV joint and will now concentrate in that area I think.
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Old 08-03-2012, 03:15 PM   #20
Stanleyf233
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OK, second hand drive-shaft (output shaft) purchased and on its way, anyone know how difficult it will be
to fit it?
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