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I have a 1991 Volvo 740 Turbo Wagon..
I started it up today (-25C outside) it was a little bit rough but after warming it up for about 15 min I started to drive.. I noticed a faint whistling sound, not a squeal but just a little whining/whistle. The whistling eventually became pretty loud and stopped when the car stopped, but started again upon acceleration. I'm unsure if this is because of extreme cold or possibly something in the turbo??? It almost sounds like if you were to whirl a rope in the air and hear the sound of the air passing by it.
Windows are closed, no roof racks, sound is coming from under the hood.
Thanks for reading !! and STAY WARMM brrrr
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After reading Onkel Udo II's thread from ages ago, and reading this... I believe I am experiencing the same thing.
The noise, which to me sounds like some kind of harmonic wind turbulence is reverberated when driving next to a wall or car, is not dependent on RPM, but does poke its head up from 15-30mph/25-50kph.
If the center support bearing on the drive shaft is the problem; not having access to a lift, I hope to do this when we have a warm day on four wheel ramps...
Item List:
1.) $20 - 6' Gear/Bearing Puller - http://www.amazon.com/Pro-Grade-18218-6-Inch-Reversible-Puller/dp/B002O16UHA
2.) $30-$1XX - Driveshaft Support and bearing (FAQ for numbers and sizes)
3.) $5-10 - pipe pieces described in FAQ
***Not Necessary*** $10-20 - Driveline Driveshaft Bolt kit - Why not? Just in case you lose one, or they are in bad condition?
My local Machine Shop charges $60/hr and is a very wise but aloof man. I may see if a local repair center can do this if I don't feel confident with the interesting way suggested in the FAQ.
This is also something I learned with bike stuff is that using the old bearing to press in a new bearing is an ideal situation! Why? the inner and outer races touch each other and can prevent brinneling/pitting inside the bearing. I don't know if it would work in this situation though.
While this is not our driveshaft, the pictures helped me imagine things much better! And honestly, they have a ton of great articles in their Literature and Catalogs section (http://www2.dana.com/expertforms/depdf.aspx?prod=JOI) such as this: http://www2.dana.com/pdf/DSSM-3264.pdf
The V70 and V40 have boot covers for various areas of their driveshaft... It would be nice to find one to cover this spot, IMO. 
What is this job in terms of time? 4 hours of work if everything goes well?
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http://www.wvcycling.net
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Of course a $5 grease gun needle adapter solves about 90% of the problems, too...
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Can you pop the seals of the bearing and repack it?
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Never bothered to try on that bearing but have done it on others.
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Try jacking up a rear wheel and run the car in gear. Perhaps then you can locate the noise. Block the front wheel securely!
Bill
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Brrr.... I used my unit converter-13 F!
I'm guessing some hose cracked from old age and giving in to winter. Have you noticed any drop in performance/acceleration? Like Uncle Udo says it's probably a hose related to turbo system. but it may be a small hose, not a big diameter hose.
I once had the small hose going to turbo gauge split under the dash and kinda fit your description. You're certain it's in under hood? Can you recreate sound by increasing throttle with hand at engine while hood is open?
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Try the easy to search Expanded Style FAQ Index **
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When I try to recreate the sound without the car moving it doesn't whistle at all. I can make a recording while driving but that's about it. It's only while moving. It does almost sound like it's coming from the dash though, it was so loud I was unable to tell exactly where.. somewhere between the dash and the hood. It's a weird sound to describe lol I do have a feel it might be something with the turbo though.. How would I be able to check?
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How to check? Good question! I'm not exactly sure because spraying water on hoses is one way, but with freezing weather that's bad idea.
Do you have a turbo guage? Is it moving into the orange zone or does it stop at about 12 o'clock no mater how hard acceleration? If yes, then likely it's this gauge's hose that runs from manifold into the car to the dashboard.
Look here in FAQ for some vacuum leak diagnosis advice: http://40mph.com/Brickboard_700-900_FAQ_Expanded_Index_Version/
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I own a Volvo or Does Volvo Own Me? Try the easy to search Expanded Style FAQ Index http://40mph.com/Brickboard_700-900_FAQ_Expanded_Index_Version/
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Is it engine speed or car speed dependent?
If the noise rises and falls with engine speed, even in neutral at a stop, it is most likely a turbo hose leak.
If the noise rises and falls with your forward velocity completely disappearing at a stop, even in neutral revving the engine, it is likely the center support bearing on the drive shaft.
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Got the chance to check out the car in park and neutral, no whistling sounds when I press the gas.. My Volvo has close to 400k on it as well, and the weather warmed up since the morning.. I noticed no difference from going up a hill or not.. Whistling gets loudest at 60km stops at around 70km then starts back up.. The sound persists while coasting but it's clear it's under the hood not by the seats..
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If it only happens in motion and then only in a specific speed range it HAS to be a bearing. You seem unwilling to accept the drive shaft support bearing consider a front wheel bearing...and after that does not pan out check the drive shaft support bearing.
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If you could describe this noise as "sort of like the sound an aircraft makes when it takes off", then I'm voting on the driveshaft support bearing.
Especially when it's cold, the lubricant (if there's any left) will not be effective, and it will squeal. Also, because of where it's located (under the car), it is very difficult to determine the direction the sound is coming from.
Even though it is beneath you, the carpets and underpadding is likely blocking most of the direct sound (at least we hope it is), and you're hearing it from whatever glass surface (windshield or side window) you think it is near.
If the car doesn't make this noise in park, or neutral, then it's pretty likely the driveshaft bearing. Luckily it's not very expensive to fix. Just a pain to get to, unless you have a lift.
Let us know.
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