Yesterday, while changing my in-ceiling air filter, I noticed something odd on the inside of my air return plenum box. Located towards the bottom of the plenum box are nails with green plastic caps, two nails on three out of the four sides of the box. Nearly all of the green plastic caps (these look like the same type of fasteners roofers use for the tar paper) are cracked and loose to the touch. A piece of one of the fasteners even pulled right off with no effort. I suspect these plastic caps were compressing the insulated lining at some point, but maybe gave in? The plenum box doesn't appear loose, but the thin black felt-like lining coving the pink insulation panel is torn slightly where each of these cap-nails are present and cracked. I wish I had a picture, but my camera stopped working some time ago. Please help!