I have an Air Arms S410, and it sounds as if something has come adrift in the pressure cylinder.

By pointing up then down I can hear whatever it is moving along the tube.

Any ideas? Or should I send it off to the makers?

you might try posting your problem on the Air Arms Owners Club site. obviously something is loose . as to where to send it, it depends on where you live (UK vs USA). google search for the AA owners Club website.