I removed the belt and inspected all of the pulleys. None of them made any loud noises when they were spinned by hand, so it doesn’t appear to be the bearings on the pulley that’s making the noise, unless they only make the noise when they are spinning really fast. The auto tensioner pulley showed some signs of wear and tear e.g. black scruff marks. The belt seemed fine. I did not see any signs of wear and tear on the belt.
I put the belt back on and turned on the car and investigated further with my mechanics stethoscope.
I noticed the whining sound starts as soon as the ac fan (at the front of the car) turns off. When I turn on the ac then the sound disappears as the fans turn on, however, even with the ac still on, the sound reappears when the fans stop spinning.
The sound seems to be coming from the right side of the compressor. This is where I biggest difference in the intensity of the sound seems to be i.e. when turning the ac on and off.