Does anyone know what could be causing this? It almost sounds like a supercharger type noise.

loose belt?

Is it only when the engine is under load, or can you hear the sound with the hood up and reving the engine at the throttle body?

Haven't tried that yet but so far it's only when I accelerate somewhat hard, not necessarily full throttle.

Just thinking that if it makes the noise with the hood open, you may be able to pin-point it better....AC compressor would be the first thought. If not, and it's only heard under load, it may not be related to the AC, and your just able to hear it at lower RPM due to the load of the AC. Just throwing things out.

^Power steering pump would be my next suspect. They tend to whine alot when going out and/or low on fluid.

Why would I only hear it when the A/C is on if it has to do with the steering pump?

a/c shuts off @wide open