what i have found causing most noises as you describe is item #15, the planetary gear set (see above picture). i have dissasembled several 722.6 core transmissions that have them damaged, one even had all the gears shattered. does the noise sound loudest below the center console area? depending on your mileage, you may be better off getting a mercedes factory rebuilt unit we have never had any issues with them.
you may be throwing your money away by just replacing the pump, especially if it is the planetary gears.