The description "tinny" makes me wonder if any of the sheet metal on the engine, or possibly a bracket, is loose and vibrating. If you run the engine again and get the sound, try touching parts to see if you can feel vibration or if the sound goes away with your fingers providing damping to the part (paying attention to keep fingers away from mooving parts). Is there good clearance to the radiator? Anything on the engine stand that might be looses and making the noise?
