I've had a knocking noise when driving over rough surfaces, and managed to investigate this properly last night.
The knocking could easily be replicated with hands at 3 & 9 o'clock on a lifted wheel - rocking the wheel left to right would repeat the knocking noise (see video for how this sounds here:
https://youtube.com/shorts/MJ_thPXHqnk). The edge of the tyre was probably moving in/out about 5-8mm in total.
I thought it was the control arms at first, and whilst those do need replacing IMO, the noise wasn't coming from them. To be honest, it was very tricky to trace the sound (especially with my duff hearing!!!, but even my wife struggled to pinpoint it). But through a process of elimination, I was pretty convinced it was coming from the steering rack!!! OMG I was thinking, this is going to be expensive!!!
Anyway, I found a thread on the net for non-M cars that mentioned tightening an adjuster, so thought I would give that a go. Whilst on those cars a special 3-pronged tool was required, my 2016 OG M2 has a plain 17mm nut at that location (see photos for location).
It didn't need much adjustment. I probably tightened it initially 1/16th of a turn, but the knocking was still present. Then I tightened it until I couldn't tighten it anymore, then backed off a bit (this was probably a further 1/8th of a turn), and the knocking was gone!!!!
Before putting it all back together, I turned the car on, and checked the steering - it was so heavy!!!! It felt like a lorry without power steering!!! It took a few attempts, but I adjusted it so that the knocking/movement was gone, but the steering wheel was still easy to turn.
Happy days! No play or knocking, and no need to buy a new steering rack!!!
Pics:
1) Shows the location of the adjuster on the steering rack (the top of a rubber bung within the nut can be seen at the end of the ratchet handle - this rubber bung did not need to be touched) - this is on the driver's side of the rack on UK cars, or the passenger side for other regions.
2) Shows the 17mm socket installed and ready to tighten - you will need a universal joint due to the angle.