@inthrustwetrust I know for sure it isn't the blend door. I replaced it a year ago by cutting through the trim by the drivers side floorboard. It's still accessible and that's not the issue, as the sound is not coming from down there. I certainly didn't hit the heater core, either, so it's not related to the blend door. I replaced the cabin air filter a few weeks ago hoping it would fix it, but it changed nothing. I'll check the fresh air plumbing, but it doesn't do it for any other vents when the fresh air is active, so I doubt it's that, either. My car is garage kept and I don't park under trees, so I can't think of how anything may have wound up in there. The serpentine belt is new, replaced it last year, and I know it's not the compressor or the condenser. It sounds like a bearing in a fan, coming from where the windshield meets the dash and can be heard from both inside and outside the vehicle.

Thanks for the help! Do you have any other suggestions? If you turn the volume up and hold the speaker up to your ear, or use headphones, you can hear it clearly in the video. It only does it on defrost, not any other vent option, regardless of if the recirculation button is on or off.

I went for the simplest repairs first but it's still making the same noise. I'm hoping to figure out what it is soon. I'm at a loss.

@mountainmanjoe I had the compressor checked. It's working fine and the air has no problem cooling or heating. Refrigerant is fine, too. It's just making an annoying noise only when the defrost is on. Sounds like a bearing in a fan that's gone bad.

if you stick your head under the hood, can you tell what area it's coming from?

@mountainmanjoe
Had the AC compressor replaced today. Still have the same noise. When the AC compressor fuse is removed, it doesn't make the noise, but there's no way it could be coming from both my old AC compressor and my new one.