When I shut off engine it sounds like the compression is being released. Is this normal?

 

this is normal and can be heard with straight piped ex even better!
even tho there is'nt any boost a idle the turbo is still probly still turning 8,000 rpm and the truck is idleing at 800 rpm. it take the turbo a minute to stop and is why it is so importnt to let the turbo cool down after you foot has been in it! when you shut the eng off, you shut down the oil suply to the turbo. the oil is there for cooling as well as lubebrocation.
it only takes a a minute, to 5 minutes, depending on how hard it has just been worked. but evan give it 30 sec is a hard habbit to make!!!

 

Thank you for the info.