1995 Toyota Previa

When I turn on the A/C in my van, it has a very loud, high pitch ringing noise, not a squeal like a loose belt. It will eventually stop, say, maybe after fifteen seconds. I will drive a little bit and it will still do it for a few seconds at a time. Doing it less frequently, but then continues to do it intermittently while I'm driving for a second or two. If I stop, shut the van off, then go do something and come back and start the van and turn the A/C on it will still do the same thing all over again. It seems like the worse of it is at start up. The A/C is very cold and works very well other-wise. Thanks, Andy