If you've ever heard a blown rear shock mount, that's the sound I have, even over small bumps. I believe its the sound of all the pieces in the strut assembly gently coming apart and going back together again over a bump.. The tiny amount of 'play' in all the components, since they're not under load like a traditional strut, causes everything to clunk.
At least, that's my best guess. I have no bad components or anything, and I've been in another (non-E30) coilover car which did the same thing. Its irritating though.
edit: with 40 or 45 series tires, pavement ridges from road construction or giant pot holes don't "bang", they "SLAM". It sounds like the whole strut tower is going to break. Of course, I have poly everything which doesn't help. :/