I was riding in the Wife's car the other day and when she turned it over a nasty clunk that vibrated all the way up through the seats could be felt. It also has done it to me several months ago just as I turned the key into the acc position. It has 55K and been put through the typical Wife pace. It sounded like the hvac door slamming shut is my only guess. Any Trailblazer techs out there?