Front knock / tap after returning to stock springs, help

[S4NoMore]

Cliff notes: front end knocks/clunks, but not so much over big potholes as during subtle body movements while going down the road. It gets worse as the car warms up. No obvious loose suspension components. Cannot reproduce by pushing down on front end.


I'm posting this for a friend of mine who has a '12 135i M-sport. I helped him go back to stock springs from H&R, because he bought the car that way but with factory shocks so the ride suffered and winter is coming (any day now...).

It also came with eibach sway bars, the tirerack kit. The front bar was groaning and squeaking, so (one at a time - bar never completely removed) each side's bushing was pulled out and greased while we were under there. That part of the job went well, no more sway squeak.

Now, this was not my first spring r&r. Once I figured out that the LCA needed to be loosened at its mount on the subframe in order to gain enough clearance to put the taller stock strut assembly back in, it seemed like fairly smooth sailing. I did load the suspension when re-tightening the LCA bolt.

The strut top mounts (bearings) looked okay, old but not so bad that we ran to the dealer for new ones. The sway re-assembled just fine, all nice and tight. The strut bracing needed to be moved to access the top bolts, we found that it was doable to take out the bolt on the tower and get enough room by pushing the brace out of the way a bit.

Anyways it all went back together well enough, we took it around the block and down the street and it rode wonderfully the way BMWs do, with no odd noises.

Well, I should mention that even before this, my friend mentioned a sort of quiet scrape-scrape-scrape noise from the front wheels under 15-20 mph usually while stopping. Before the spring job I had a hard time hearing it, but it was perceptible. The brake pedal did not change it. We saw nothing wrong with the brake dust shield, no obvious evidence of any interference.

After my friend got home following the spring swap he called me to say that there was a new noise, almost a knocking kind of clunk. It happens while driving straight, independent of brake and throttle, moreso while turning and/or hitting small bumps. Strangely, it usually does not happen when going over a large/nasty bump, like the ones that make you cringe. I can hear it clearly through his car's microphone.

We double checked everything when we put his winter wheels on. I found the strut pinch bolt to be not fully tightened, and I was hopeful that was the culprit as the strut rotated and its nub clanged against the knuckle slot, but the noise persists. It can almost be felt in the floor, it isn't very loud but without music it is incessant and very annoying.

I read about the strut bar developing slack and I thought for sure it had something to do with it, but in taking off the cowl covers we found everything to be tight.

I feel bad, I know how frustrating unknown noises can be. Please share if you have any experience with symptoms of the upper strut mounts going bad, or the LCA/ball-joint going bad or being improperly tightened. I did not replace the LCA bolts, but I later read that they are stretch bolts. Is that possibly related? Thanks for any input.