If you think its transmission, could be the front pump but usually the pitch and noise level will change with engine/gearbox speed and gear selection.
You could also remove the drive belts on the engine and then spin each pulley for bearing noise. These can also make wierd noises like this. Also check power steering pump, alternator and water pump.
This noise is very difficult to determine, I recommend lifting the vehicle and listening for the noise then. Or place on jacks, run engine with gearbox in Park and listen for noise then. You will be able to get close to the gearbox to have a better listen.
Good luck and look forward to your feedback on problem