California model will have the dreaded air recirc valve and the trap oxidizer -- check for a loose intake hose between the turbo and the air filter housing. When the throttle is closed, the air recirc valve will dump boost into the intake hose, and make a squealing/swooshing noise if the hose is loose. This also means you are sucking in dirty air, so you need to check pronto.

You will also get the noise if there is a loose blowby line there, too.

If you have the trap oxidizer still in place, you need to go to a dealer and discuss replacing it with a catalyst.

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!