Loose scope rail...use some blue locktite on the screws, they always vibrate loose.
That should solve your problem. I'd recommend that you remove the other 2 screws holding the receiver cover in place. Now carefully wiggle the stock with receiver cover upwards to remove.Remove cocking lever screw from bolt... Clean threads with q-tip and degreaser then tighten with blue locktite. Also check barrel retention grub screw with 2mm Allen key or t8 torx for tightness.