ok, i got a 94 fullsize blazer, had it about 4 years now and since day 1 i`v been getting a loud thumping/flapping noise from under the dash passenger side but only when i start or shut the truck off((turning the key from on to off)), it does it for about 15 seconds each time every time, just annoying, but never seem`d critical to fix. after 4 years of it maybe i just formed a deaf ear to it, but hoping for any ideas so i can fix it. thx for any help