Knocking noise . . .

[oldgeebee]

I'm hoping someone might be able to point me in the right direction to track down the source of quite a loud knocking noise which appears to come from the rear end of my S3 and which is most noticeable at low speed. It seems to occur when taking left bends but not right bends and can occur under light braking but goes away if I brake heavier. Knocking speed is road speed dependent.
I replaced all 4 CVJ's which has cleared up the clanking noises but only made the knocking noise more apparent - it was there before I changed the CVJ's.
The wheel bearings seem in good nick - no play with the wheel off the ground. I haven't checked the prop-shaft UJ's yet but wouldn't have thought they would be "bend dependent".
There can't be much left apart from the diff but I'm hoping it's not that (how to check?).
Grateful for any suggestions!
GB

[tozerman]

Worth removing the rear shockers and checking the upper and lower bushes for condition and also that the bolts are correctly tightened up, if you have Gaz Golds then check the spherical bearings for any play, good luck.

cheers.....Tony..

[oldgeebee]

Hi Tony - I haven't checked anything going "up and down" yet 'cos is sounds like it's something going "round and round". Would bushings give a noise frequency proportional to road speed?
GB

[tozerman]

If it is a knocking noise then it can only be 2 surfaces touching, I don't really get the round and round bit, but certainly the trailing arm bushes could also cause the problems you've got.

Cheers.....Tony..

[oldgeebee]

I'm thinking round and round 'cos the knock is regular and proportional to road speed - like a "stone in a tyre" type regular.
I should be able to get out there this afternoon and check the shock mounts and trailing arm bushes - so we'll see!
GB

[tozerman]

OK I see now, if it was my car I would be checking the tyres carefully and also removing the drums and having a good look there and checking the balance of the rear brakes, I say this because once my rear shoes were out of balance and when braking I got a noise similar to the one you seem to be getting ! good luck

cheers.....Tony..

[oldgeebee]

Shock bushes and bolts look OK as are the trailing arm bushes.
I had the drums off before since the noise gets worse with light braking but everything seems to be tight and where it should be with nothing looking worn. Not sure what you mean by "balanced".
Tyres look OK as well.
Still a mystery I'm afraid
GB

[Alan Whitaker]

Hi All
could be a bent drive shaft?

ALan

[oldgeebee]

Hi Alan,
I didn't check the drive shafts for true when I had them on the bench when replacing the CVJ's but when I "spin" the shafts with the car up on axle stands and the wheels off there is no obvious wobble.

One curiosity is that near the offside trailing arm bush there is a hole and washer right next to the bush bolt and a couple of holes in the body mounting bracket close by. The nearside doesn't have these holes so what could have been mounted there and is it significant that it's missing?!!
GB

[GreenV8S]

CV joints would have been my first suggestion but you already addressed that. The next most likely candidate would be a pitted race on one of the wheel bearings. This may only show up under the right combination of lateral/longitudinal loads. Before you get too far, make sure it really is coming from the rear. noises like this can travel through the chassis in unexpected ways - it would be ironic if you stripped the rear end down and then found one of the front wheel bearings was collapsing.

[tozerman]

oldgeebee said:

Not sure what you mean by "balanced".

Basically that both rear brakes are equal in the amount of braking force being produced, when i had a similar problem my os rear brakes were providing more braking force than the ns which gave me a knocking / juddering when braking ! Hope this helps

Cheers.....Tony..

[tvrgit]

oldgeebee said:

One curiosity is that near the offside trailing arm bush there is a hole and washer right next to the bush bolt and a couple of holes in the body mounting bracket close by. The nearside doesn't have these holes so what could have been mounted there and is it significant that it's missing?!!

Doesn't look standard at all - don't know what it could have been though. Only thing I can think of is that somebody has made a small bracing bracket to bolt through the suspension mount and attach it to the body mount plate, to reinforce the joint between the chassis and the suspension.

If it was me, I would be checking the chassis very carefully in that area, just to make sure the mounting isn't deparating from the chassis - if it was, you might get a knock from the rear suspension under certain load conditions.

[aero340]

Have a look at the rear rubber mounting bracket on the diff these can split and allow the transmition of noise ...

[imccar]

Doubt if this is going to help much..... however, I've had this exact same problem on and off for a few years (S3).... no effect on handling, braking etc, but bloody annoying low speed "rotational" knocking particularly bad at lower speeds (higher the speed, higher the frequency of the knocking to the point that it "disappeared") and going round left (IIRC) bends. It would come and go every few months. Investigations into the whole of the "back-end" whenever it occurred revealed nothing, but usually made the knocking disappear for another few months.
Bizarrely enough the last time it happened was just before an accidental grounding of the chassis on a mates drive since which time it has not returned (over a year ago)..... said it wouldn't be much help, just to let you know investigations into the brakes, bearings, suspension, bushes, etc.. revealed absolutely nothing.???

[oldgeebee]

Thanks for all the ideas - I've had another look around this evening but nothing obvious.

aero340 said:

Have a look at the rear rubber mounting bracket on the diff ...

Had the wheels off and "tested" the mountings with a crow bar but they seem pretty solid.

tvrgit said:

If it was me, I would be checking the chassis very carefully in that area...

I've checked the chassis in this corner and it looks solid - no obvious cracks around the trailing arm mounting brackets

Greenv8s said:

The next most likely candidate would be a pitted race on one of the wheel bearings...

The crusty old mechanic at the Ford place where I tried first for the CVJ's said that an ungreased spline on the axle stub could make a cracking sound as it rotates under load. This might be something to try but I don't fancy the 240Nm torque on those nuts!

imcar said:

said it wouldn't be much help, just to let you know...

sort of comforting to know that it went on so long without falling apart! Problem is, I probably won't get the wife into the car until it stops scaring her with all that nasty knocking. Come to think of it that might be a blessing in disguise ..
GB

[AndrewS2]

Similar road speed dependant knocking noise earlier this year on mine proved to be some pretty shot and one almost ceased prop shaft UJ's. Took it off and had it completely refurbed by North West prop shafts, refitted and problem solved. Mine was worse at lower speeds, and under decelaration... BUT, I'd convinced my self all sorts of things before I gave up and went to work on the prop. Left hand bends, only in full moon when in third gear and with a 12 stone passenger...the works

If you've not had the prop done before, and you think it's suspect, can't do any harm and its not to expensive.

Good luck hope you figure it out.

Andrew

[oldgeebee]

Hi Andrew,
I was certainly thinking UJ's but didn't pursue that when it seemed to get worse when turning left and/or braking. They're a bit of a br to get at so might need to be a garage job unless there's an easy way to test them? I wouldn't mind doing the job myself but wouldn't want to drop the exhaust (and everything else that implies for a DIY dodderer) only to find out they were OK!
GB

[AndrewS2]

There is a picture of me somewhere having finally managed to remove the prop, after 7 hours of sweat, blood and a lot of language that made my mum shut the kitchen window!!! It is a faff, but not complicated, there are some great instructions on here if you do a search. You just need some spanners, a socket set and the car on ramps...

Oh and brute force. Lots of it.

Don't know a way to check without removing. If they are originals though they will go eventually, so if you got it off and they were fine I'd consider it good, preventative maintenance?!!!

Others on here have much more technical knowledge than me though... just my suggestion!

Andrew

[ketvrin]

+1 for the prop shaft uj's and the pain to get it off... However there is a much easiernway that Mark1234 detailed AFTER I removed mine see his website for details

K

[oldgeebee]

AndrewS2 said:

If you've not had the prop done before, and you think it's suspect, can't do any harm and its not to expensive

I like your thinking Andrew - but...

ketvrin said:

However there is a much easiernway that Mark1234 detailed AFTER I removed mine

I think my aversion to tackling this myself came from reading Mark's website - dropping exhaust, diff and gearbox!?
GB