A friend at my job asked me about this because of our similar trucks. He does not think it is coming from the engine, but it makes this noise within minutes of starting the truck, as though someone slaps the truck open handed.
I will advise him to the insite shared here.

 

Probably blend door closing

 

Yep, mine does it just about the time my mirrors are clearing the shop doors. Scares the heck out of me every time.

 

Can he fix it to eliminate the bang?

 

Maybe but I think they all do it.Even my L-9000s done the same only louder

 

They all do it, some are just a little louder. Takes gettin used to.

I imagine it's something to do with the vacuum building for the first few seconds the truck is running, before it closes.

 

Used the scare the **** out of me also, thought I hit something backing out of the garage (kids bikes, skateboard, etc..). Mine doesn't do it anymore as long as I put the heater/AC selector in the off position before shutting the truck off. Try it - it works for me. Occasionally I will forget to do this and the bang will be there after startup. But as long as I put that switch in the off position it will not happen.

 

Thanks for the replies. I have shared this with my friend.

 

Turn off the Heat/AC before starting. It will then happen when you turn it back on. Jim