This is a new one for me I've owned VolvoS since 1985 and I've never heard this noise coming from the engine.
it's not a skipping belt.
It's not consistent as it will crop up at any time and it does not change intensity based on the rpm's.
this page won't accept my vid so here's the link to my YouTube channel and the short clip of the sound.
Any assist will be appreciated
Best regards
https://youtu.be/g7uxiXQDBJQ

 

That noise sounds like the breather box (I believe its called.) I had same on my 2008 S80 3.2 sometime between 150,000 to 175,00 miles. There is a plastic/rubber piece inside that ages and just doesn't perform as needed.

To test if this is indeed your BB, start engine, open hood, pull dipstick out a couple inches. Does the whistle completely disappear? If so, its the BB and an easy fix. I was traveling so had no tools and paid $500 US ($200-300 ish for part, $200-300 ish for labor) back in 2015 or 2016. The rectangular part (which I can see in your video) sits on the top of engine closest to firewall. I believe it has 8 small bolts that secure it. Unscrew them, unplug an electrical connection, and its off quickly. Place new part of. reattach and secure. Done.

*** Again, i didnt actually do the replacement myself so i could be wrong about this so continue to research it beyond my comments. ***

Good luck.

 

Hello from Eastern Canada.
Isn't that a lovely frequency!
I can understand dh2of3's comment but it is hard to believe it's the 'breather box'. However stranger things have happened.
A vibrating object, caused to vibrate by the air passing over it will change frequency in relation to the air velocity.
But if there is no frequency difference when the engine is at higher rpm (therefore more air movement ) then the physics of fluid dynamics are not working in this incidence. I would love to hear the noise at higher engine rmp, Maybe it does change pitch slightly.

 

not uncommon for faulty PCV systems to create a whistle. Quick test is to pop out the dip stick or crack the oil fill cap to vent the crankcase and see if the noise goes away.

 

UPDATE: The sound is intermittent and does not change if the engine is revved or at higher RPMs.
However, if I pull out the dipstick enough to break the seal, the sound stops. It appears to have significant vacuum from the DS, so someplace there's a blockage that restricts the inflow of air causing it to "whistle"
Is the Breather Box attached to the firewall or the engine? I need to RnR the plugs anyway, so it seems like a good time to attack this issue.

Currently car shopping, so need to fix these little issues B4. I've enjoyed the Volvo life for the past 38 years and they were like old comfortable shoes that you don't want to throw away.
But age is setting in and my 2nd childhood it looking to the cars I couldn't obtain in my younger years...Panamera, A8 and S550...(slightly used and drastically depreciated will be the game!)
Thanks all for the years of support and help.
JC

 

It is on top of your engine. You can replace just the diaphragm if that is the issue. Different part depending on T6 or 3.2 trim.

Diaphragm is the part that normally goes so you don't need to spend hundreds on a part. The diaphragm is $40 or so. I circled the diaphragm and highlighted the pcv box.