AC compressor noise

I found a thread back from 2005 that related to this question, but it didn't really answer my question completely:

How loud should the AC compressor be? I know that the noise is related to the speed, and that the speed changes with the demand, since the compressor is variable speed and electrically driven.

My car is only a couple of months old (2009 model). I usually leave the "climate control" (a.k.a., "auto AC") always turned on, with the cabin temperature set around 71 F.

Today the temperature was in the mid 70's, and I was running some errands. I stopped for lunch about a mile from home. When I got back in the car, the interior had heated up somewhat, so the climate control was calling for more cooling. When backing into the garage the ICE was off. I noticed a sound like someone using a handheld electric cake mixer, but louder. It was coming from the engine compartment. I found that it was coming from what appeared to be the AC compressor. As the cabin temperature dropped, the compressor ran slower and got somewhat quieter. If I turned the AC off, the noise stopped, and if I turned it back on, the noise resumed.

It was not very noticeable when the ICE was running, but you could hear it once you knew what to listen for. With the ICE off, it was very noticeable.

The AC seems to be working just fine (blowing cold enough), so I don't think it's low on fluid. I would think that if it was low on fluid that it would not be able to cool well enough.

Thanks in advance for your feedback.