My box does the same. Raises the question; what`s a rebuild on these things..? I "rebuilt" mine, replaced bearings, seals, rings etc, but I think the only way to get rid of this particular problem is to shim it in some way or replace all internals. I haven`t taken it apart again, but the only thing I can think of is there must be wear to either the wormgear or housing ,allowing the worm to push up before engaging. I tried to adjust this, but that didn`t help.
I`m a blacksmith, no mechanic so maybe I did something wrong. hmm...Maybe I should try the BFH approach