I have had fan clutches go bad in exactly this manner. They appear fine when you give them a spin by hand, but when spun by the engine ( especially during cold operation) will tend to wobble up to 1/2" front to rear. If you have close tolerances on the fan shroud to begin with, this will most likely cause the clanging. When I've had them go bad, they have stopped the major wobbling when warmed up. Give it a visual, and check the front to rear spread of the blades (wobble)when the engine is running versus hand spinning. If in doubt as to what it really is, take the fan off and start the engine when cold.
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