[ChevyGuy]

If it disappears when your rev it slightly, I'd take a look under the hood. It might just be something rattling (perhaps a heat shield) and it's related to the exact engine speed when the A/C is on and not anything to do with the A/C itself.

Then again, it might be the belt tensioner starting to go.

 

[ChevyGuy]

It's mostly noticeable when I'm stopped at a light in-gear. It seems to lessen ever so slightly when I drop it into Neutral - but it's still quite noticeable.

This is puzzling. Normally, the engine speed will go up slightly when you shift from drive into neutral. If the noise is tied to engine speed, I would expect it to get louder, not softer, when you shift to neutral.

Not that uncommon in cars (in general). Something is loose under the hood and it vibrates at an exact RPM. Putting it in neutral shifts the RPM just enough so it won't vibrate as much. Heat shields are frequent culprits - large chunks of thin metal bolted down in only a few spots.

But it could be something on the drive belt. It might be worth popping the belt off and giving things a spin by hand to make sure the bearings in the water pump, alternator and tensioner are not "crunching".