Noisy Air Conditioner

I have a 2000 Jeep Grand Cherokee Limited with 37000 miles. When ever I turn on the air conditioner I start to hear a noise (sounds like one of those little plastic birds you fill with water and blow in to).

The air works fine but the noise is driving me nuts!

It seems like it gets louder when there is stress on the engine (right before changing to the next gear). However, the noise is constant only when the air is on.

Any help would be greatly appreciated.

Thanks,

Rob