The fan should spin up and roar when you first start up, not thump.

I suspect it's either installed incorrectly (if that's possible) or defective. Check for loose bolts, etc -- the fluid will all be on the bottom at startup, so if it's loose, it will thump until the fluid spins out.

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!