You may have something that's dragging a pulley enough to make the noise. In my case the power steering pump hangs for a fraction of a second when starting and it makes a loud squelching noise - I'll replacing the pump soon. This could be caused by anything on the belt (whether timing or serpentine) - if you have a Harbor Freight near you they have a stethoscope made for locating sounds fairly cheap (bought mine for $3 several years ago but I think retail is about double that). It will help you locate the sound.