Just the ever curious side of me wanting another answer. I have noticed in a couple of different WWII movies that the chief engineer of German Type VII-C U-boats will listen to the engine with a rather peculiar looking device. It looks almost like a funnel with a long metal stem. He puts the large end up to his ear then the end of the stem is pointed at the engine at the cylinder head area between the large rocker arms. What exactly is he listening to or listening for? You can see the engineer in the movie Das Boot doing this several times in the movie and in the TV series Das Boot the engineers have done this in several episodes.