Canadian CH-47C (long time ago) was ground taxiing out after major maintenance. The Flight Engineer, known to be a very picky guy was walking up and down the cabin, listening. He suddenly said - Shut Down Now! and front seat crew promptly complied. When all was stopped, they asked -Why?
Response - I heard a noise I didn't like.
Panels opened and a screwdriver was discovered in the upper driveshaft tunnel busily working it's way through the main driveshaft.
Maintenance officer as well as several senior maintenance personnel were in deep, deep trouble as tool control was suppsed to be very strictly complied with.