I have a 2004 3 2.0L at 144k miles that has been making this knocking noise for several years. I first noticed it when turning out of a parking garage and the wheel dropped an inch or two; I thought I was going crazy. Now, fast forward a few years, it's constantly banging around on slightly wavy/bumpy roads and especially when driving over potholes or man hole covers that are below the street level. The best I can describe my sound is like a bushing that has disintegrated and it's a metal-on-metal clanking around coming from the passenger side wheel well/shock mount area. It doesn't matter if I'm braking or accelerating and it seems to get worse when the wheel is turned slightly to the left on a sweeping curve. I don't feel it as much as I hear it; it's frustrating to no end and I actually wish something would just snap/fail so that at least I can identify it.
The suspension on my 3 is practically overhauled except for the most inconvenient: sway bar bushings. In the last 20k miles, and not because of this noise, I have installed Koni Orange struts and replaced all the hardware around it except for the spring, Moog endlinks (front and back), lower control arms (front), tie rods (inner and outer), and the passenger CV joint most recently. None of those made any difference.
I've read endless forums on this and different fixes seem to be the issue but most of them didn't do the work themselves so no one said specifically what component failed. My front bushings look intact and I'm not sure what it would be hitting to make such a noise, but that is the only thing that I think it left that would cause this.
You're not alone, but if you do resolve please come back and share what fixed it.