I know, describing noises is the hardest on this or other forums. But here is mine

If you ever blew over the top of a bottle(s) the sound changes with the size of the bottle. Mine is like a mini bottle, very gentle. more on the high frequency side.
First I thought it was windnoise from the rear driverside window or doorsill. All is tight. The puzzling thing is it appears more when it is cold, it can stop completely and is quiet for some time, then re-appears.It is more constant when someone sits in the rearseat (I was asked if I am transporting a squirrelcage). It is "somewhere in the back". It is off when the car stops, is not appearing if going over a bump only (suspension squeak), appears more when making a left turn. It does not seem to be a metal noise, like a turning metal against another.
Drives me nuts

 

I was just about to ask you if it's a UL, and then saw in your signature it is. I initially thought of UL having a different hvac system for the rear, but I believe the only difference (related to the HVAC) is that you have a separate filter + UV lamp for the rear air.

Could it be related to your HVAC system fan? Is the noise still present if you shutoff the HVAC completely? When was the last time you checked on your rear seat air filter? Could it be your ARMREST air ducting to the back seat? I know there's some air ducts that run through the center armrest, as the air makes it's way out the 2 vents that basically point to the center rear passenger. Try opening your armrest while you're driving and hear the noise. Obviously be careful driving while you do that? You could also try closing those 2 vents to the center rear passenger and see if it changes anything? You could also change the mode of those vents from face to floor.

Have you ever sat in the back to try and place a better ear to it? I know nobody else will want to fix our cars BETTER and more accurately than those of us who own and drive them. (I have a rattle somewhere on the rear decksill that I need to troubleshoot by having the GF drive me around. Just haven't gotten around to it yet.)

Could you try moisturizing/lubing all the door seals? The thick foamy door seals that is, and the difficult to see seals at the bottom edge of the doors. Not saying totally Shin Etsu the door seals, but maybe dampen a sponge in something simple like Armor All protectant and see if that has any change?

How about the EXTERIOR beauty/finishing seals on the rear doors? The exterior seals I'm talking about, the outter most seal that follows the perimeter of the door, and makes the physical seal with the painted portion of the door opening to the rest of the door? I know on mine, the outter most edge of that weatherstripping, about 1mm, has dryrotted and was hanging like a piece of thin linguini. I took a razor blade and had to cut it off. The hanging piece of black linguini happened on both rear doors, around the same sharp curve near the door handle. Hopefully you know the area I'm trying to describe.

That should give you enough to work on for starters, lol.

PS----keep that ultra cold Arctic-like air up there, north of the USA's northern border!

 

thank you so much for the suggestions.
I had a mechanic sitting in the rear, but he (we) could not hear a thing; like going to the dentist and the pain is gone.
From your comments I now believe you are right, it must be related to the HVAC system. I had the rear a/c line disconnected because the highpressure line to the rear had holes and was prohibitively expensive to fix; you still have ac in the whole cabin without having both on. BTW our repair rate is 150/hr. You are very quick in 4 figure territory plus taxes. Would the lowpressure line still be active sucking in air somewhere?
In any case I will investigate your suggestion thoroughly when the weather is more favorable. These days you cannot be distracted a bit from the road
yes, the sound is like air rushing over or through a small opening.
The doorseals are maintained with Gummipflege and are soft and pliable. No sound from the windows. By turning my head to the rear it seems like coming from the door or from underneath the floorpanel and both a/c lines were running under the left rear passenger floor where the holes were discovered (that is another story)

 

How old is the mechanic? If he's in his late 40's or 50's he might not be able to physically hear higher frequency sounds. Have someone young sit in the backseat and see if they can localize the noise...

 

I am definitely over the 50s range and I can hear it. The problem is locating it

 

People's hearing loss varies with individuals and at a different rate. You can't just put an age number of hearing deterioration. Some young people have damaged their hearing from years of very loud earphones. Some older people have very good hearing (I am not one of them). Speaking from experience.