drivers door wont shut! & exhaust leak maybe...

In my 2000 chevy blazer, the inside door panel is loose/ coming off. I took it apart and found out that some plastic washers that held on the metal bolt that holds on the whole door panel, are broken. Has this happened to anyone? I just dont wanna take it to the dealer and have them charge me 300 bucks for an all new door panel. Also when i shut my car off (2-3 seconds after the engine is cut) i hear a loud bang from the catalitic converter, like metal being bent. I ignored it and now it seems to do it more often, when i shut it off and when i turn it on. Anyone wanna buy a blazer?