motor noise infromation

[mr slate]

I hope you guys may shed some light on this problem. My 96 super duty seems to have a noise similar to a belt slipping coming from the drivers side of engine. It only seems to occur at acceleration and only from 2000 rpm up and gets louder. Going up and down hills it fades out doing down hill then picks up when the uphill starts again. I have changed the belt and no change was heard. The vacume pump and power steering pump seem ok. Brake booster works ok and no loss of power has been noticed. Currently in excess of 200k miles. Any help or ideas to locate this sound would be great