Dear IH and Garvin,
Thank you for your replies.
His words were literally " When the slide goes to the rear ". . . and from what you are both saying it seems that this is not something that you have encountered.
I've repeatedly watched a Youtube video of someone cocking a FWB 90 and it does not seem to knock when it is cocked.
He has had it 15 years and says that it is accurate ; but having never handled one before myself, I could have it in my hand and not know if this knocking sound is the sign of a future issue or not.
I think that this is the last air gun that I will buy so I guess it would be better to go with one that does not knock.
Thanks.