Oil pump proplems

My son let his oil get low on his impreza now he has a lite knock after it warms up but it is a harmonic knock around 2500 rpm and then it sounds ok we replaced the timing belt a few months ago and thought it was the components from that it was not. it is coming from behind the crank gear.
the oil pump is behind the gear and looks like it could be wiped out if run dry.. Drained the oil and cut the filter not a spun bearing but 4 small pieces of aluminum in oil pos from the face of the oil pump. Any help would be great. Thanks.