Starving your engine of oil doesn't have to take long to damage internals. It took my engine about eh...10 minutes of starvation to jack it up. I would get a stethoscope as mentioned above and pinpoint. Probe around the bottom of your block (bottom 4" or so) and if that is your problem, it's a bearing. Also probe around all other areas as well. Sound travels through metal VERY well, so for instance...a noise at the top audible without aid could actually be a problem on anything, anywhere that is attached to your block. To put it in perspective for you...my water pump failure was audible unaided around the valve cover, the only way i figured it out was had a shop run a scope on it. Ordered new part and sure enough the mechanism was seized up when i pulled it off. Could you maybe get an audio/video clip? We may/may not be able to help more than just throwing suggestions out there. We're merely giving you starting points.