"Pop" on shutdown?

[donh54]

Took the Limo for a run to see if I've cured the coolant issue (dumping a heap of coolant after a run around the block). Cautiously optimistic about that, after (hopefully getting rid of an airlock).
After the test run, pulled up in the driveway and shut down, and got a "pop" from the engine as it stopped. Started and shut down again, same result.
Any clues from the brains trust?
PS. It's a 4.6 GEMS.

[Roverlord off road spares]

is it a pop coming from the intake? like unburnt fuel back firing thru the intake.

[donh54]

is it a pop coming from the intake? like unburnt fuel back firing thru the intake.

Sounded a bit like that, but seeing I haven't disturbed anything except the coolant expansion tank (lifted it about 45mm), though I did have to unplug the connector under it to give me some room. They seemed to plug back in OK, though.