Well, I searched the gearbox section quite thoroughly, and much was covered, but no clonks! The difficulty from 3rd to 2nd can be more accurately described as initially an unwillingness lifting out of 3rd, followed by a CLUNK when it finally shifts into 2nd, and this can be felt as a light smack on the boot upper when it CLUNKS into 2nd. A similar but lighter clunk can be felt shifting into 1st.
I have assembled my gearbox many times, but never noticed any syncro rings. The dogs on the individual gears of the constant mesh box are fine though.
Cheers,
Otto in Denmark