Knocking sound over uneven surfaces

stannie203 · Post by **stannie203** » Thu Mar 23, 2023 9:45 pm

Hi, after taking the plunge and buying an e89 Z4 - dream car - I've had nothing but problems, and disapproving looks/words from the wife.

In 2 months I've had issues with the roof not closing (think I found a loose hall sensor with no fixing barb) replaced a rear spring and now this... It's actually making me feel really disappointed and almost regretful for plunging for my dream car.

I'm getting a knocking noise coming from the rear of the car over bumps/uneven roads (which are terrible in North Yorkshire!). Sounds like it's in the boot but probably lower down in and around the suspension.

Weirdly, unlike with the spring, it's hard to hear when I have the roof or windows down. The spring sounded like a bag of spanners and was quite loud with a window down, yet this issue still sounds like it's more in the boot. And feels or sounds central rather than related to one particular wheel (as per the spring).

Sound like anything familiar?!

Help/reassurance that I've not done the wrong thing required!

Pondrew · Post by **Pondrew** » Thu Mar 23, 2023 10:03 pm

stannie203 wrote: ↑Thu Mar 23, 2023 9:45 pm Help/reassurance that I've not done the wrong thing required!

Don't worry too much, the E89 is generally a good car with very few expensive or hard-to-find common faults.

What engine/ spec do you have? IE 20, 23, 28, 30, 35 or 35is. M-sport or SE spec?

If the noise is in the general vicinity of the boot area, first thing is to check everything is secure in there. All the gubbins under the lift out carpet; battery, roof motor, tool kit (!) etc, etc.

If you provide the information, someone will find the problem even if by a process of elimination.

DMike · Post by **DMike** » Thu Mar 23, 2023 10:12 pm

I have zero knowledge on e89s, but generally a loose or broken exhaust & silencer hanger could cause knocking in that area — and is easy to check.

stannie203 · Post by **stannie203** » Thu Mar 23, 2023 10:24 pm

I've given the boot a good check and everything feels really secure. The battery has a bar strapped over it and something near the foot of it to stop it moving.

Unless it's roof mechanism moving that I can't get to, but it probably feels like it's lower down than that, even though I can't hear anything out of the window.

Underneath the car I can see one exhaust silencer hanger (I think that's what it is) and the exhaust generally feels really solid if I try and move it a bit at the end. Everything I can get my hands on under the car feels really solid (albeit a bit rusty, but that's to be expected at 11 years old, right?)

As I said, one new spring a week ago and the other rear spring was replaced not too long back before I purchased. Although, knowing what a broken spring sounds like, it's not that.

Pondrew · Post by **Pondrew** » Thu Mar 23, 2023 10:41 pm

So is the knocking sound like something is loose and hitting something else?

stannie203 · Post by **stannie203** » Thu Mar 23, 2023 11:17 pm

It's so hard to describe. It sounds like it's coming from the middle rear, lower part of the car. It's quite a dull thunking noise over uneven roads. Sometimes the smallest rift in the road causes a thunk, but if I manage it over a speed bump then it's fine. Or if I press down on the boot to rock the car then there's no noise.

I'm on run flats, so I imagine that doesn't help (not something I will change until I absolutely have to or the wife will kill me!)

flybobbie · Post by **flybobbie** » Thu Mar 23, 2023 11:54 pm

I had this when i bought my car. (BMW were a waste of time, yet it was an old BMW bulletin that gave a solution).
Took 6 months to solve.
I removed the rear spring on the right, washed the rubbers top and bottom.
Covered them in talc. Refitted and i rotating spring on fitting by 45 degrees anti clockwise.

The sound, like a rubber mallet hitting underside of boot went away.

I believe the barrel springs pinch the rubbers and rebound causes the knock.
viewtopic.php?t=129915

B21 · Post by **B21** » Thu Mar 23, 2023 11:58 pm

Could be a worn bush in the ARB links..among many other bushes,,,

flybobbie · Post by **flybobbie** » Fri Mar 24, 2023 12:00 am

Greasing anti roll bushes helped for a day, noise came back.

Pondrew · Post by **Pondrew** » Fri Mar 24, 2023 12:00 am

stannie203 wrote: ↑Thu Mar 23, 2023 11:17 pm I'm on run flats, so I imagine that doesn't help (not something I will change until I absolutely have to or the wife will kill me!)

Mine is also on RFTs and doesn't do this, so don't think it's that. Trying to run them down but they are tenacious gits!
Only way I can think to ascertain where it's coming from is to find somewhere (generous) with a ramp and get it up and wiggle everything underneath to see if anything is loose. Unlikely to be the diff or driveshafts, they are pretty good and you would notice an issue everywhere if that was an issue.

Edit: posted before the above.

john-e89 · Post by **john-e89** » Fri Mar 24, 2023 12:08 am

I have exactly the same knocking you describe OP, it's a worn bush for sure, I've had it on a ramp and everything feels ok but one of them has gone, unfortunately it's a bit of a minefield as I've used a pry bar to try to find it but to no avail, you can usually feel which one has gone. Having said that I've not come across a knackered bush on these Zeds before, wouldn't think it's any different to any other BMW but at the moment all I can do is strip things down until I find it. Its not a top mount as I've swapped the rear shocks and mounts round to test them. Once it's found it'll need the other side doing too, these early cars are now in the realm of needing some money throwin at them I'm afraid. Not much help to you I realise.

flybobbie · Post by **flybobbie** » Fri Mar 24, 2023 8:44 am

All i can say about bushes is that my car is 13 years old and 98k miles, bushes haven't worn.

Rockhopper · Post by **Rockhopper** » Fri Mar 24, 2023 9:08 am

Anti roll bar drop links are favourite for knocks going over bumps. Cheap and easy to change as well.

stannie203 · Post by **stannie203** » Fri Mar 24, 2023 10:22 am

Thanks all for the replies.

One thing which sounds plausible is the suggestion that the newly fitted barrel springs pinch the rubbers and this rebounding causes a knock - that the springs need rotating by 45 degrees. Would there be any tell tale visual signs of this pinching that I can see without taking the wheel off? Would I expect to see some kind of rubbing away of the black paint of the spring?

I didn't notice this 'underside of boot being hit by a rubber mallet' noise (as someone so accurately put it!) before the spring was replaced. Trying to decide whether a) I didn't notice because the snapped spring was masking the noise, b) the replaced spring fitting is causing the issue, c) that something else has gone kaput since having the spring replaced and it's no coincidence at all!

john-e89 · Post by **john-e89** » Fri Mar 24, 2023 11:21 am

flybobbie wrote: ↑Fri Mar 24, 2023 8:44 am All i can say about bushes is that my car is 13 years old and 98k miles, bushes haven't worn.

Mine is on 118k, I guess they have to start at some point. Probably should have mentioned mine started shortly after fitting KWV coil overs which may well have flushed out a wearing bush due to the higher poundage springs.

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