hi,
i recently bought a 2000 V70 w 120k miles, most everything about it seems good for its age & milage.
after a couple long vacation drives this summer i noticed what sounds like brake booster vacuum leak or whine. when i start and/or shut off the engine and sometimes while inside the car while driving i hear a related or similar moan/whine when the A/C cycles with the AUT setting on the climate control.
im pretty familiar with my older '95 850 wagon but the few places i have looked/checked for a worn or leaking something-or-other i haven't found it yet... i felt certain it was brake booster leak but... now im not so sure
any suggestions/guesses on where else to check?

also, i have heard the AWD can be a problem as it ages, what should i look for with that?
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-woodsy; 245k 1995 850 wagon; 123k 2000 V70

i think i'm gonna go get my length of hose and listen around in & outside the car and make my wife nuts starting and stoppind the engine for an hour. see if i can track it down.
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-woodsy; 245k 1995 850 wagon; 123k 2000 V70

Bob is correct. When my ATF got low, it used to do an awful noise and drove me nuts. Seems the previous owner 'dumped' ATF and refilled it to the full mark without the engine running! The tranny was 2 quarts low.
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My name is Klaus and I am a V♂lv♂holic

Mine is definitely a vacuum hiss/whine.
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Obama, One and Done!

I have an 01 that makes the same sound. Sometimes it will make the whine when shifting from drive to park. I think its in the blend air door area of the climate control. I'll get into it and report back. I can hear mine the most clearly when I shut the engine off.
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Obama, One and Done!

i'm thinking it's one of those shift/blend doors or something like that too. i had to replace the heater core a couple weeks ago but after the repair had no time to poke around under the dash or hood to finish trouble shooting this whistle/whine item. i haven't looked into it too deeply yet but it looks like vacuum is still used for lots of the climate control stuff in the 2000 V70xc.
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-woodsy; 246k 1995 850 wagon; 125k 2000 V70

There is NO vacuum controlled anything in the cabin of your car. The cruise control is on its own vacuum line and has nothing to do with the engine.

The climate control for the XC should be electronic, if it has an 'auto' position for the blower and vents.

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My name is Klaus and I am a V♂lv♂holic

yup. my climate control is auto.
and, the whine/whistle seems louder outside and it occurs whether the AC or climate control is on or off when i start OR shut-off the car. i'm puzzled.

...but the sound of it sure makes me think of vacuum issues and i does seem to correspond to the climate control stuff when i operate it.

the ATF and all fluids have been checked and are proper... runs well otherwise, but the whine is sorta janky and makes me wonder if whatever is doing it it'll quit on me some day when it's cold and rainy and i'm hours from home in the dark with nothing but a dead cell phone... ok, im not THAT worried , but i want to find out what it is...

grr... i need to get out the ol' stethescope and listening hose, sigh
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-woodsy; 246k 1995 850 wagon; 125k 2000 V70

My climate control is manual. I've begun to notice that I can also hear a whine/hiss right after I take my foot off the brake pedal. The brakes appear to be fine though. I can put my foot on the pedal and it is hard with the engine off. If I hold the pedal steady and start the engine, the pedal sinks about 3/4 inch and holds pressure steadily. On removing my foot from the brake pedal, I can hear the moan/whine and the hiss for a couple of seconds.
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Obama, One and Done!

Not too sure what kind of noise it is, but perhaps it is time to clean the idle air control. The IAC comes on when the key is turned to posII, you can verify the noise without turning the engine on. Clean it with throttle body cleaner and a toothbrush, shaking the TB cleaner around the insides and then pouring out the sludge.

If the brake vacuum leaks, the vacuum line is on the left fender? The best check for that is to apply the brake hard before start and if you feel the pedal sink about an inch after engine start, all is well.
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My name is Klaus and I am a V♂lv♂holic

Sounds like your A/C is low on refrigerant. They will make a whiney or moaning noise when low. Note, your car does not have an idle air control valve, only the 98 model year had those. Yours has an electronic throdule module instead of a throttle body and IAC valve.