2002 Ford Explorer Rattle at 1000 RPM

My Explorer has started to make a rattle. This only happens when the engine is at 1,000 RPM's. I've narrowed down the area where the noise is coming from to the under side of the engine on the passenger side. I also bought a stethoscope and believe it's coming from a box where the lowest pulley is attached (cannot find a diagram to know what this pulley does). I've just changed the serpentine belt and still it makes this rattle. No check engine light and the vehicle performs normal. Any help is greatly appreciated, thanks.