[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?

[HoosierDaddy]

This is a "long shot".... Alternator? I would think it is not under load when the key is off.

[whatsnewell]

Yep, sounds like the gear driven alternator. They tend to have a rattling sound.

[Glenn and Laureen]

Thank you, Hoosier and Clarke. The alternator idea makes sense. I will check that out tomorrow.

[bestgenman]

It could be the starter not disengaging.  I’m thinking it is a 40MT Delco which has an external solenoid to force the drive gear into the flywheel.  If it isn’t returning on it’s own, then the drive gear is being turned by the flywheel or the teeth are hitting.  If it hangs in the flywheel , road speed operation will turn the started drive gear extremely fast, faster than it can endure!

It’s not a common occurrence but does happen occasionally.

[Glenn and Laureen]

Genman wins by getting closest to the actual problem. A mechanic confirmed for me that the starter is not disengaging until I turn the ignition switch to off. The root cause is that the ignition switch circuit continues to send power to the starter after the key is released. Does not stop sending until the switch is moved to the off position.

So, good news for me. No alternator or starter problem and no engine problem. Do need to replace the ignition switch and a really old relay in the back electrical panel that serves the starter.

Thanks to everyone for your advice. Laureen and I learned a lot and also met a good guy who is a diesel mechanic and all around Mr Fix It for future reference.