Summary

The air filter in our home is mounted in the ceiling and has about an inch of play. Every time the heater started the filter was lifted up, and when it stopped the filter dropped back down on the metal cover, making a loud noise.

I first fixed it with some paper towels as shown in one of the photos, but then I made four little spacers that clip to the air filter's cardboard case. Now the filter is held in place at all times and there is no play and no noise.

The spacers were modeled quickly in Autodesk Fusion 360 and printed on a Formlabs Form 1.