Bizzarre intermittent sound

[AirSpeed]

Originally Posted by old Chevy guy

Could it be the fan hitting something? maybe a sloppy motor mount.Dont get why it would go away after shutting it off no matter what it was.

I can't figure out why shutting it off makes it go away either. The fan clears the shroud and the motor mounts are good. I can't find any interference anywhere. If it didn't go away once I shut off the engine and if it matched the engine speed I would swear its a rod knocking, I've had a couple engines with a bad rod bearing sound just like it but this just doesn't match the engine speed and it goes away. I drove it around yesterday trying to get it to happen again but no luck (or maybe it's good luck!) I'm sure it will happen again when I least expect it.
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