1999 zx2 noise coming from engine

Hope you can help my daughter has a 99 zr2 that just started to make a loud noise coming from the engine when put under load, or given a little gas. it sound like it is coming from the passenger side of the engine compartment. When stopped at a light it is very load but once you get moving the noise seem to go away. The engine runs fine on and off the highway. I've taken to my mechanic and they are scratching there head. They remove the belt trouble stayed. They are going to start tomorrow at the timing belt and look at that area. Any suggestions.
Thanks Paul