I searched a couple threads to find a similar issue but nothing posted before quite matches what I'm hearing. Reddit and youtube didn't turn up results either.
I'm driving a 2017 XC60 T6 AWD, 31k miles and the recent 30k mile manufacturer's maintenance visit didn't turn up any issues. The only service the car has had was to replace the brake pads on all four wheels about a year ago. I got the car used about three months ago and have been driving it almost daily since. It passed my mechanic's inspection with no issues and the test-drive didn't turn up the problem below. I've recently noticed a persistent whine that sounds like when a mosquito or a gnat buzzes into your ear coming from the area of the front passenger side of the car. It is hard to pin down if the sound is coming from the engine block or the wheel well. My wife claims she can hear it coming from the driver side but I've only noticed it from the passenger side. It only occurs loudly when I've accelerated up to around 60-65 mph however I can sometimes hear it at echoing off of fences/guard rails/surfaces at slower speeds if I drive with the window down. If I increase my speed, the whining gets higher in pitch and vice versa.
There is no change in how the car handles, the turning is smooth and there is no vibration. I have tried unscrewing/tightening the various fluid caps under the hood and I have also tried playing with the climate fan speeds and temperatures, opening and closing the air vents, muting or turning off the radio; all to no avail.
Volvo service couldn't hear it however my mechanic was able to reproduce the sound and said they would set up a future appointment to take another look.
On other forums I've gotten suggestions that it could be the power steering pump (my fluid level was fine), the alternator, wheel bearings, or cracked hoses in the engine's cooling system; my mechanic checked out the steering pump but not the others.
Here is a recording of the sound that a friend was able to get and clean up for me: