It ACTUALLY sounds like popcorn "rattling" in a kettle, like at a movie theater. Remember GMs' old 6.2 Diesel engines in pickups? It is a "Legacy" noise of the old style P/C headed Diesels. They used an odd head design that had a "pre-combustion" chamber in the head where the injector is. It basically started the combustion process, due to the smaller area where the fuel/air mix entered the head, and then, the flame ignited the rest of the mixture. ( I think that I got it right. My memory is getting dated.) It has been so long since I have driven anything with an old Diesel that the noises are unusual now.