[Glenn and Laureen]

We have a 1984 Newell with a 6V92 Detroit Diesel engine w/turbo that we bought in April of this year. The engine has run very smoothly and extremely quietly since we bought it. Have driven from California to Texas with no unusual engine noises or issues. Last night when we started it to return it to storage there was a loud clatter/clanking noise coming from the engine bay immediately upon starting. Had sort of a metallic sound. I turned the engine off and looked in the bay for smoke, broken belt, anything - but found nothing wrong. I then searched online about possible causes. Feared it might be a broken rod, damaged piston or similar issue. Then I thought it might be the diesel equivalent of knocking (detonation) but don't know why that would suddenly happen. 

Today I discovered that with the engine running (and making some louder than usual noise) I can turn the ignition key to off and remove the key. I do not hit the kill switch. When I turn the ignition off the noises stop immediately and the engine is its old, quiet self. What could possibly explain this? Is the starter not disengaging after the engine starts? It starts fine immediately. I also drove the coach about 30 miles at speeds up to 65 mph today and everything was fine re the engine (sounds, temp, pressure, speed, power). What mechanical thing in the engine bay stops working when the ignition is turned off?