Would be interesting to try the stock sprocket and see if it disappears (I know you don't have one now). The only thing I can think of since you have a relatively new chain is that perhaps the cush drive in the new hub is culprit. If it is exceptionally rigid or has too much play it could lead to a slack issue that can compound with a tight spot and creat a rather annoying slap. The only real way to determine this is to change one component at a time.
This could explain the "at all speeds" issue because even at a steady speed the cush drive is bouncing back and forth just do to rotational inertia and friction differences etc... I am not an engineer but have had enough physics (for the inevitable science driven drivel that will undoubtedly appear on this post).