Hi Tom, I don't know if this is of any help. But I notice a chopping/knocking noise on my Hegner 18v just before the blade pops out of the upper blade holder. When I hear it happening, if I hear it early enough I can check the upper blade clamp and sure enough the blade has slipped down in the holder. Then I just put it back in and load it back up and then everything is hunky dory.
Like I said, I don't know if this helps or not. Hopefully it does though.
Jim