Have you replaced the o-rings on the center "bolt" on the oil filter? These are not in the filter box, and when they get hard, they leak and reduce oil pressure to the lifter gallery. Usually causes excess noise on startup from oil draining out, but I don't see why it couldn't also cause warm noise instead.

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!