The cocking of the Model 66 is nothing like as "smooth" as when cocking something like a Walther LGV etc and it does sound a bit rough in comparison as the two small gear wheels wind their way along the gear teeth and the two pistons return to their cocked position - but I would not describe it as sounding like metal grinding so it deserves some further investigation.
We are lucky to have some real Giss experts in the UK who are also a members of this forum so I would suggest you give DM88 or wonky donkey a PM to see if they can help to answer your question for you.
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.