[Racer20]

Final Edit: Here's what fixed it. Spoiler alert: it's the steering gear. You can skip most of the gobbledegook below unless you want to fall down the same rabbit hole I did trying fruitlessly to solve this issue.

-----------Original Post--------------

There are a bunch of threads on this issue already, but I've started down my own rabbit hole of figuring out what's going on myself rather than relying on hearsay and paid-by-the-job dealerships to solve the issue.

The problem:

A loud metal-on-metal clunk that sounds like something heavy is loose in the suspension. Isolated to the drivers side on my car.

Vibration felt through the floor and pedals, noise sounds like it's coming from the strut tower. No vibration through steering wheel.

In addition to the noise, it seems that there's more harshness being transmitted through the drivers side suspension, but this could also just be the road surface.

Occurence:

Driving at low speeds over sharp, upward-protruding road imperfections, causing the suspension to compress quickly. I have a test bump near me in an empty parking lot that creates the noise consistently. Reversing over my test bump turns it into a sharp rebound drop rather than a compression hit, and the noise is not present in reverse.

Driver's side only: Hitting the test bump with the passenger side wheel at the same angle and speed does not produce noise.

Occurs in all suspension modes, including when the damper is unplugged (locked in full stiff). Noise severity gets worse with increasing damper stiffness.

Noise occurs regardless of steering input or brakes on/off.

This problem seems to occur mostly on cars that have had suspension work done and mostly on the driver's side, but I haven't verified this thoroughly at all. Chime in if your experience is different.

Other people complain about clicks, creaks, and noises while steering. These noises might be related, but I don't have any of those issues. Mine is specifically a heavy clunk over sharp impacts, more prevalent at lower speeds.

My car:

2015 M3 19" wheels, EDC, (June 2014 production)
CS suspension swap (= ZCP dampers and rear sway bar)
MP HAS
CS EDC/GHAS/MDM/EPS Coding
(Ride height set to BMW recommendation and alignment set to factory nominal settings)

The basics:

My initial suspension installation was done by me*, with all new hardware and proper torques.

*Except the ZCP rear sway bar. The shop that did this work also found that my drivers side front lower control arm was bad, so they replaced it. I didn't ask any questions or get the parts back at the time because there was no noise or other problem. The noise started occurring gradually about 1000 miles after all this work, earlier this summer (I did the suspension retrofit last summer/fall and when I put the car away for the winter, everything was fine.)

Here are the first few things I checked, and didn't find a smoking gun:

1. Everything feels and looks tight and in good shape. I can't find the noise by hitting anything with a mallet. I've looked at everything under load with the car on ramps and with the suspension at full droop with the car on jack stands.
   
   
2. My sway bar end links are not preloaded at curb position. (Some people claim that correcting a preloaded bar fixed their issue, at least temporarily, but this isn't verified and it did nothing for me.)
   
   
3. I've dyno tested the drivers side damper and found it to perform exactly like when I put it on last year. No degradation whatsoever. I can feel a clear difference in ride when switching modes; there's no indication that the EDC system is malfunctioning. Edit: This is not true, see post #21 for update.
   
   
4. There has been some discussion in other threads about adjusting a bolt on the steering gear to take up slack and eliminate play. (Some people claim this fixed their issue, at least temporarily)
   
   Don't. My friend who works for Bosch in steering gear development said not to touch that bolt, that there's a specific procedure for setting the preload and doing it yourself could cause heavy wear and screw things up. Take it to the dealer if you think the steering gear is an issue.
   

Prime candidates/interesting findings:

The upper strut mount:

(Some people claim that replacing this fixed their issue, at least temporarily, but this isn't verified)

1. My upper strut bearing (plastic thing between the spring and top plate) had a lot of play. I replaced it (just the plastic piece) with a tighter one from a ZCP with just 6k miles on it. The road texture isolation improved on that side of the car and the steering effort reduced in right turns, but the clunk is still there.
   
   
2. The washer in the upper strut mount that the strut rod sits against and clamps to has some witness marks and fretting damage, and I can feel the edge of the witness mark with my fingernail, indicating that the washer material has been compressed.
   
   
3. BMW changed the upper strut mounts from 3 bolt to 5 bolt attachments in late 2014. I can't think of any good reason to make this change except to solve a joint stiffness issue.
   
   My suspicion is that the noise is caused by the upper mount not being fully clamped against the strut tower sheetmetal. Your car clearly has the 5bolt mounts, but who's to say how effective the change ultimately was? You have 5x8mm bolts while I have 3x10mm bolts, so bolt stretch could still be an issue.
   
   Based on how the clunk sounds, the lack of any other clear root cause, and the odd change to the mount design, I think this is one of the more likely root causes.
   
   As bushings and tires wear, the loads that go into the upper mount could i crease to the point where the joint integrity is compromised.
   
   This theory would be difficult to test for and even more difficult to fix though.
   

The Lower Strut Mount:

1. Based on advice from other posts, I re-torqued this bolt and the noise went away for about an hour and came right back. This is an aluminum to aluminum clamping joint, which can be sensitive to torque and damage/deformation, so maybe that first re-torque/over-torque was a bad idea. (Some people claim this fixed their issue, at least temporarily, but not verified.)
   
   
2. There are some oddly shaped witness marks on the strut can where it interfaces with the lower clevis on the spindle that indicates that it might not be seating properly, but I've re-assembled this joint several times and the clunk never changes. The marks could also just be from the installation or disassembly process itself. I discovered the other day that there's a special tool BMW specifies to pull the strut into the spindle housing to a specified torque, but I can find the tool anywhere to buy.
   
   If anyone has machining capability and could make this tool for me to test, PM me.
   
   Either way, I'm going to clean and reassemble this joint with new hardware exactly as specified by BMW.
   

Control arms/balljoints:

1. Again, everything feels tight here, and my thrust arm only has 1000 miles on it. But if the other stuff doesn't work, here are some possible things I'll look at:
   
   
2. The BMW procedure says not to allow the balljoints on the control arms to exceed 25deg of travel during any assembly processes. The easiest way to install struts (the way I did it, or anyone else not following or aware of BMW's requirement) violates this, so it's possible they are damaged, even though they feel tight. I have not loosened these joints for further inspection yet. Once I get through my work plan with the strut, I plan to replace the thrust and tension arm, then duplicate any replaced parts on the passenger side for symmetry. (Some people claim that replacing control/thrust arms fixed their issue, at least temporarily, but not verified)
   
   
3. BMW procedure also says to torque the control arms at curb position with load on the suspension. Not doing this can cause the bushings to wear very quickly and change the way loads are distributed through the chassis. I don't know if the shop did this, so I'll be checking it once I get through my current testing related to the strut.