AC noise

Whenever I turn on my AC I hear a whining noise (I think it sounds like a belt). It is constant as long as the AC is on. What could it be?

Well, all of the belts are under the hood.
Have you lifted the hood in order to determine where the noise is coming from?

If the belt that drives the A/C compressor is making noise, that could indicate several possibilities:

The belt is loose, meaning that you need a new belt tensioner
The belt is glazed or worn, and needs to be replaced
The belt is slipping because the A/C compressor is on its last legs and is about to seize

If the noise is not coming from under the hood, then it could be coming from the motor of the HVAC fan.

Have you tested it without the A/C being activated?
Try running the fan on the same speed(s) in both vent mode and in heat mode, and see if you still hear the noise.

Check under the hood, and run the fan check that I suggested above, and then come back to this thread to report your findings. Also, please tell us the model year of this Sienna and its odometer mileage.

I lifted the hood, and the noise is coming from under the hood. I tested it with only the fan on in both vent and heat mode, and the noise is not present. It only makes the noise when the a/c is on. This is a 2006 model, with 64,629 miles on it.