If it is a 4-cyl engine,the most common cause is the belt tensioner pulley.It also is dependent on vehicle speed or engine speed.Does it make the noise when sitting still and revving the engine? It's all a process of elimination.
I agree with bmdouble, remove (have the belt removed) and run engine to see if the noise is now gone. If so there is a problem with the belt drive/driven system. Turn componets by hand (key off) to see if anything shows up. Feel, look & listen while turning.