I finally found the source of my clunk. I know the OP doesn't have 2.5" shocks but nonetheless it's possibly related.
I decided to re-valve my front Icon
shocks about a week ago. I did a lot of reading and I talked to the Icon guys about doing it. Anyhow, when I finally went to discharge the shock before opening it, I discovered that one shock had leaked out all of its nitrogen pressure. I called Icon and asked how this could happen. They said it's not common but could potentially happen once in a blue moon. I found most interesting their comment that if the shock did lose its nitrogen, a KNOCKING noise would result.
So I recharged the shock after installing different shims and a new charge port. No more knocking noise! I would imagine any mono tube shock could experience this issue if it became discharged. Simple way to check is remove the coil and try to compress it by hand. If it doesn't rebound back to full length there is no nitrogen. Hope this helps someone. Good luck!