It is common for this to be the auxiliary belt tensioner bearing or the belt itself. You can check with a long screwdriver, place it near the bearing and hold the other end to your ear, if the sound is loudest near this bearing that is probably the problem.
Other possibilities are the injectors getting noisy or the timing chain being slightly slack. The timing chain is on the left hand dude of the engine under the top cover so try listening arround that area. The injectors are near the front where the inlet manifold is so listen in that area as well.