I installed the HKS HIPERMAX RS coilovers. They feel great but everytime I go over a bump, even a tiny one, I hear a loud, muffled noise come from the rear suspension area. I can't isolate it but it sounds like somebody dropping a book on a table(muffled). It sounds like it's coming from both sides as well. It's especially bad over speed bumps. Again they feel great and I have adjusted the firmness to the point where they are really soft right now and I still hear it.
Any ideas what it could be?
My drop is about .80 inch and I am all aligned. Coilovers were the only suspension mod. My tires are 30 series, maybe thats what it is?
Thanks in advance..

 

Is everything tightened down properly? Make sure there are no loose nuts or bolts. Also pillowball mounts aren't going to help much with noise suppression vs the rubber mounts.

 

Very strange I know others have the HKS and it's silent. You may have faulty shocks, but yeh check everything again to ensure it's tightened properly, especially where the rear spring is seated too.

I had problems with my Tein CS, my sound was more like rattles. Anyways my turned out to be faulty shocks, something to do with the shims. Tein repaired under warranty, you may want to contact HKS as last possible option.

I know the feeling, u pay good $$ so u expect everything to be perfect.

Good luck!

 

my hks hipermax rs is brand new and my shop(ZCG) has isolated the problem to the rear shocks. They replaced both shocks with temporary ones and that solved the problem.
That might be your problem if its a popping noise.