I had one a couple weeks ago that drove me crazy trying to locate it.
Long story short, the spring that holds back the bendix in the starter had broken and the starter drive gear would just sneak out and tap on the flywheel.
The stethascope got me close and after removing the flywheel cover the noise got louder, when turning the flywheel by hand I could hear it... still took me a bit to realize it was the starter drive!