You can glue some of the rubber backed commercial carpet tiles to the outside of the unit. I've done this before on steel wheel chair lift doors that really made a noise when the clanged closed. It was amazing on how the rubber backed carpet silenced the noise--and it looked good too!! Works similar to the silencer on a bow string by absorbing the vibrations. I have no idea of the legality of shooting in your city--but I wouldn't think it'd be illegal to glue carpet to a bullet trap? Still you may want to check--it's legal in sticksville, where I prefer to live.