There is a plastic duct on the back side of the radiator so the fan draws air across the entire rad. If it has slipped, or the engine mounts are dead (more likely, that!) the fan blades can make contact with the plastic. Sound can vary from a faint tapping to horrible, something is getting beated to death, depending on amount of contact and position.

Should be obvious, will likely be on the bottom of the shroud if the engine mounts have gone flat.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!