I just installed ST coilovers on my e93 IS and had some very bad clunking noise coming from under the hood. Apparently, when lowered, the springs will hit themselves and possibly rub in a way that causes a clunking sound. After tons of research and help from Rich at GTB Performance, this is what finally stopped the noise.
I had another thought with the Coilovers. Often as well as the springs hitting, there are a couple of other things that can cause the clunking. I would check for any loose bolts, especially on the control arms. Lower control arm bolts that are loose can quite often cause this issue. Also, double check to make sure the springs are seated correctly against the top hats. If they aren't, there will be play that could be causing the clunking. No accessory piece for the spring from ST. I'd suggest some PVC tubing from Home Depot, cut in half lengthwise. Also, give the other suggestions up there a try and let me know how it goes.
First of all, I would like to thank you for your help and research in tackling this problem. So, I believe the PVC tubing worked. I heated up two 10 inch long 3/4's wide PVC tubing and slid it on around the top of both coils (not having to cut lengthwise). I tightened everything back up and as of two days driving, the clunking is gone. I canít begin to tell you how happy I am. Thank you for the suggestion and the great customer service.
This almost made the noise go away completely. However, I could still feel and hear the same noise but very faintly. So, I tried one last thing.
The single most important part of the whole install is how you tighten the nut that holds down the spring, bearings and top-hat. For me, I thought tightening it as much as I could before installing back on the car was the end of it. You must hit this nut once more after mounting the springs back on the car with the wheels mounted and load bearing. Trying to torque this nut before the springs and shocks or mounted back on the car with load is useless. From under the hood, I used and electric impact gun that I rented from AutoZone and hit that nut on both driver and passenger side till the nut started to scream help. Jumped in the car, road down the block and bam, no more noise. 3 days later and still no noise. Hope this helps you.