Sounds like drivetrain noise. It's probably from the tooths on the gears as they are transferring force/torque on the "wrong" side of the gear teeth. They are designed to transfer torque under acceleration and might make more noise when decelerating as the teeth are engaging on the opposite side of the tooth. Race transmissions or transmissions that are for heavy duty use have more straight cut gears. Straight cut gears make more noise but don't generate the longitudinal force that wants to push the driving and driven gears apart from each other (one gear wants to go forward and the other rearwards, creating a lot of frictional losses and stress on bearings).
An automatic transmission uses planetary gears and don't operate in the same way as a manual transmission.
My guess is gear noise.