Sounds to me like you are over shifting. I have a Mk II on my 620 also. In order to shift to a lower gear, I have to over shift then move the shifter back a bit, otherwise a lot of noise. I think it has to do with the gear ratio on the freewheel. Closer gear ratio requires less overshifting. For wide ratio freewheel, try the newer Shimano 6 speed freewheel with ramps, much easier to shift.