Hi!
Can't think this is possible, because the combustion chamber dome in the head is much higher/deeper than the glow plug is long...and remember that you are using a medium length glowplug (like all Japanese glowplugs).
Are you sure you hear a metallic clicking sound only when you have the plug removed ?
Is the engine real tight ?? Most modern ABN and ABC engines aren't so tight compared to the old K&B engines 6.5cc engines of the past so they don't squeak when the piston reaches the top dead center.
Can you feel if the piston/cylinder fit tightens when piston reaches the top? Try wiggling the prop back and forth past the piston top dead center...is there much play?
If it is it ...might be the big end in the connecting rod that has a little too much play...
There are shorter glowplugs available but all modern engines use long or medium length glowplugs and so does yours.

Regards!
Jan K
Sweden