Odd double clunk in TM AEG

[SSPKali]

Just got the TM M733 and on testing noticed that there is a double clunk - first is the piston head hitting the end of the cylinder but then there is another - possibly the piston bouncing? Loose spring guide? Chopped spring.

 

FPS is +/- 3fps and shooting 295fps on 0.20g BB so it is working fine. Motor height is set correctly and everything seems tight (as tight as the polymer body TMs can be!) Gears sound like they are shimmed ok, no rattle or whine.

 

Not come across this before!

[SSPKali]

Update - the gearbox was well shimmed and all running well. Spring wasn’t chopped and firmly fitted in the piston (no shifting it!) and not rattling on the guide.

I am swapping out the 6mm PLASTIC bushings for some steel ones and shimming as A) they won’t last long with an M100 spring and Warhead motor and B) they are really tight on the gear shafts! 
Ported cylinder and new piston & piston head on their way too as the old ones were perishing and only 1 metal tooth on the rack