Very loud engine squeal

I have a 2002 4wd Explorer Sport, 83000 miles. 2 weeks ago driving home there was an extremely loud squeal coming from the engine. It started while I was on the expressway and continued for the next 30minutes only stopping while I accelerated. The next morning it stopped but started again in the afternoon when my air conditioner broke. I assumed that was the problem and I needed a new compressor. It air works now but the squeal came back yesterday. It only does this on my way home in the afternoon. Yesturday it would only stop when I was braking or stopped, today it is extremely loud again and stops only while I accelerate. I tried turning my air off and it still does it. My gauges are all fine and I have new brakes on my car. Any ideas? I would like to have an idea before I take it in some where.
Thanks!