2002 F350 7.3 Diesel BANG on startup

[slingman]

2002 Ford F350 7.3l Diesel after vehicle sits overnight or for longer than one half hour there is a bang noise heard from the passenger area it happens only once sounds like a rubber mallet hitting the floor board.
I have heard it could possibly be a concern with a heater or a/c mode door but have not heard if there is a repair available.
Thanks

[bigcountrysg]

Usually it is the vent door slamming shut when you turn the key on. The door is operated by the vaccuum system. So when turn the key on the system looses vaccuum and the door shuts making a noise. Mine does it as well on occassion.