After a few second burst on full auto when I release the trigger there's a klonk.... I know it could be anything but any ideas, could it be the piston just returning ?
The piston returning after releasing the trigger is a small "clack". If the anti-reversal latch is missing then it's a longer sound as the gears move the motor.
That sound is because your bevel gear has 4 teeth that the anti reversal latch can catch into and when it starts to reverse, it can move a bit until it catches.
If you want to get rid of it, there are bevel gears with 6 or more of those teeth.