This knock can happen to any Rockshox Monarch. Let me show you what’s going on.
I had to make a 6mm soft jaw to pull apart the rebound rod to see the compression needle inside. Anyway in this picture you can see an extra 0.5mm of space to the right of the compression needle o-ring. It’s harder to see the rebound rod o-ring but it has the same problem. The knocking around inside that extra space is what causes the problem. How do we solve it?
There are a few ways to solve the problem. You can either double up the o-rings, or install 0.5mm backup rings behind the o-rings. I’m not aware of such tiny backup rings so I just made them. Another option would be to machine a proper 1mm o-ring groove somewhere else on the rebound rod or you could even deepen the existing 1.5mm groove to fit a 1.5mm o-ring. There you have it folks. Get rid of that play in the o-ring grooves and the knock goes away.