Your first step is to remove the serp belt from the pulleys (you don't have to actually remove it from the car- just make sure it isn't wound around the crank pulley) and see if the noise is gone. If the noise is still there, remove the AC belt and try again. If the noise still happens, stop the engine and pull the fuel pump fuse so nobody runs it, to avoid the bent valves that you will have when the timing belt breaks.
If you don't hear the noise with the serp belt off, or AC belt off, then go to each pulley driven pulley and spin them, looking for a dry bearing.