Need help from DUers with car repair knowledge.

[NoGOPZone]

Started my car yesterday and heard a loud whine from under the hood. Raised the hood and it seemed that that the noise only happened when the A/C compressor was engaged. Went back in the car and felt the air coming from the vents. It was fairly cold. Turned off the A/C, and the noise stopped and the air from the vents got warmer, as expected.

I've not used the A/C again except to run a pressure test on the low side port. It was reading at 20 with the A/C on max. I havent heard the noise with the A/C off.

I'm going to leave further diagnosis and repair to a pro, but of course don't want to get tricked into unneeded service. Does this sound like it needs a new compressor, a simple recharge, or something in between? The more I know going in the better chance I have of an honest repair.