[65comet]

If from back of engine. Starter not fully disengaged? Something contacting the flywheel? Problem in the tranny? Main bearings going? Rear main seal rubbing on crankshaft? Cam bearings? Lots and lots of things could be going on. Maybe try getting a mechanics stethoscope to probe for a more exact location of the sound. They are pretty cheap at discount stores like Harbor Freight or similar. Plus sounds like to echo around the engine bay and what you hear from above as the back may in fact be coming from the front. It can be very frustrating chasing down noises.