Squeling Noise

I have a 1.8L 2004 Toyota Corolla with 70,000 miles. For at least the past year I hear this squeling noise coming from the engine. It accelerates a decelerates with the speed of the car. Sometimes I hear it. Sometimes I don't. It seems I hear it most in the morning for some reason. I do, however hear it in the afternoon and evening on occasion. Because the sound speeds and slows I would think it's the belt. Some people have said that the belt just needs tightening. Any ideas?