If this is at idle, and it turns into a vibration/hum/grind when revved, it's the exhaust on the cross member. IMO.
If this is at idle and increases with acceleration, it could be a cracked fan shroud, or a loose shroud, hitting the fan. Turn the engine off and see how much you can move the shroud....did I say engine OFF. Not worth losing a finger.
If your A/C is on...or blower motor, it could be the squirrel cage bearing, allowing the cage to wobble and contact the sidewalls inside the blower area.
This doesn't sound like engine, tranny or t- case noise to me, it sounds like some accessory part out of whack and intermittently contacting either a rotating or vibrating piece.
I would climb under and start pushing and pulling on sheet metal, exhaust pipes, checking pipe clearance at the cross member, hoses or hard lines.
contacting moving parts. This could be something super simple.
Or it could be a water pump bearing, check the weep hole for leaks.