Get a long screwdriver and with the metal tip, place it on the water pump. Put your ear right against the handle, in effect listening to the screwdriver and you will hear the sound. By placing it on different parts ie PAS pump, Sump etc where the noise is the loudest, you will find the problem.
Pump bearings normally let go if belts are too tight or its had enough and you may find a small leak from the pulley area. If you can, you can try starting the engine without the PAS belt fitted to eliminate it as to where the noise is coming from?