Rear A/C noise

2000 Montana with rear A/C, 94k miles. Air seems to work well enough, but when the compressor is on I hear a whine or howling come from the rear of the van. The sound transitions from low-pitched to high, and usually sounds like air moving thru a tube. Sometimes it sounds like one of those slide whistles they used to use as a sound effect in an old movie. It happens frequently, and can occur at any speed, but more prevalent (or noticeable, anyway) at idle or low speeds. It's audible from the outside, as well. My dad bought it in '01 or '02 used, and it has made this noise the entire time, to the best of my recall. He says he has brought it to the dealer w/out success. Any ideas?

dont blame the dealer for having trouble with this one. i had a 01 in with what seems like the same noise that you are describing and it took me at least 3-4 hours to finally figure it out. the rear thermal exspansion valve is sticking. i replaced the valve and the van never came back

Thanks - Maybe that'll do the trick.