Since three or four months there is this zooming sound when running the car. I can't really place it, it seems to come from under the dash, but because of the engine noise I can't exact tell where it comes from. If I change gears the sound changes a bit and it stops 4/5 seconds after shutting off the engine. The sound is not always present; I would say 9 out of ten.

Does it have something to do with the vacuum system, it looks like everything is working fine?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Sounds like your fan clutch. Try a search on the topic. Many threads on repair and diagnosis for your viewing pleasure.

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Orland Park, IL
1985 300SD 215,000 miles

Hello Jorn
Can you record the sound and post the audio file?
This is shot in the dark guessing:
#1. Heater-A/C blower motor obstruction, I had one MB get a cigarette pack wrapper into the blower squirrel cage, zooming/roaring sound.
#2. Heater-A/C blower motor squirrel cage cracking and flexing like an airplane propeller, zooming/roaring sound.
#3. Engine cooling fan; fluid coupling failed/engaged, I have encountered this on so many vehicles, that it is a routine check/test on all vehicles for me, recently it saved several customers a radiator, the fan blade was cracking off.

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ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Thanks, I will look at those suggestions. I never hear the sound when I'm looking in the motor bay, it really seems to come from under the dash, with or with out the heater running. My AC is not working....another project I can use some help, maybe later.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Hello Jorn
Chuckle, Chuckle, LOL.........
AC = Alternating Current.
A/C = Air Conditioning.
I have seen automotive engineers make the same mistake.
Have a great day.

Who said he was making a mistake? ...he could have a 12VDC to 110VAC inverter on his car that isn't properly hooked up...

:p :p