Rattling backglasses are usually caused by shrunken, decayed, or lost foam pads.
These pads are applied to the white wood behind the backglass. Usually they
are applied in an area where the backglass has only opaque areas. A good
replacement for this is mylar-backed, self-adhesive weatherstripping. Use of
anything without mylar backing may cause the foam or part of the adhesive
material to interact and perhaps damage the backglass.