So the noise went away after adding oil?
Lots of places for oil to leak, you'll need to do some detective work. How badly does it "mark it's spot" ? Does it blow blue smoke under some conditions? What are the conditions?
Marking it's spot indicates a leak directly from the engine, so you'll need to track where the leak is. Common places (not in order) include the rear main seal (especially if you are running the stock PCV system and the rings are not sealing perfectly), rocker cover gasket, front main seal, oil pan gasket, push rod tubes, fuel pump gasket, generator mounting bolts, and coil mounting bolts. (The generator and coil bolts go all the way through the outer wall of the block, so if one fell out you'll get a major oil leak.) Less common but still possible are the cam bearing locating bolts, the plug behind the camshaft bore, and the screwed plug in the back of the cylinder head. Oh yeah, don't forget the oil filter head itself, which has multiple places that might leak. A common problem here is that someone failed to remove the old canister sealing ring when doing an oil change. Oddly enough, having two rings installed almost guarantees a leak.
If the exhaust is blue primarily under heavy throttle, then oil is getting past the rings and cylinder walls. If it's blue primarily when backing off the throttle (eg shifting gears), then oil is getting past the intake valve guides.
As far as further damage from running low on oil, my approach would be to continue to drive it and see what other symptoms develop. Chances are that there is some damage or accelerated wear from lack of lubrication. But if the part was destroyed, you would already know it by now. So you may have only shaved (for example), 10,000 miles off of a 150,000 mile lifetime (meaning you'll rebuild the engine for other reasons before the rapid wear on this part becomes an issue).