I have a 2006 Ram 3500 megacab completely stock with about 165k miles.

Over the past several months I've noticed when I shut it down there's a sound just as the engine stops turning that sounds like high pressure air is released. It didn't do this before... then started doing it occasionally last summer... now it seems to do it all the time.

I'm not sure if it's related... but I've noticed a little soot around the manifold/head at the #6 cylinder exhaust port. I've not noticed any cracks in the manifold... not yet.

Any ideas on what the noise could be? Should I be overly concerned?

Thanks,
John.