whininng problem

hey guys, my friend has a duster just like mine with a 360 727 and he is having a problem with his transmission. He says that its making a whining sound at around 30mph and just whines. At one point his fluid was low, so he added some and he said it stopped and then started again and the same thing happened when he changed his filter yesterday, nothing for a bit then it came back. Any clue what that could be?