Noises are hard to isolate, from a DIY'ers point of view, one of the most useful tools is a stethoscope contraption available at the auto parts store with a long metal probe.

With engine running and all safety bases covered (no loose clothing, eye protection, etc), start touching the probe in the areas you think the sound is coming from. You should be able to find the part as it will be very obvious, only in your case you may need the help of an assistant to rev the motor and possibley have the car raised as well.

Good Luck!