While I don't know what the sound is, I can help you diagnose it. Buy, borrow or 'liberate' a cheap stethoscope. Take off the chest piece and substitute a thin, 12 to 18" copper tube. You can listen to precise points to try to narrow the sound down.

On my '72 Land Rover, I can pull off the valve cover and listen for individual tappets. It's also invaluable for finding tiny vacuum or exhaust leaks, water pump, or alternator bearing noises.

Good luck!