I've noticed that if the truck sits for a week or so without being driven, every now an then right after starting, there will be a loud clunk in the engine bay. I have to assume it is from the AC compressor. I always leave it set on auto. There is nothing else that I can think of that would make this sound. The truck is in park and it occurs right after starting. No noise after or right after a re-start. Will only occur if truck was sitting for a week or so.