I put one of these spacers on and so far, no knocking, even after climbing a long incline into the wind. I won't declare the problem fixed tho until i've gone several days without hearing it.
the 1mm spacer left me with just enough room in the dropouts.
if it does start making noise again i'll just get a new freewheel, something is screwy about the threading on this one (no pun intended)