You say that the fluid looks good and smells fine. Is it low by chance? A noise like you are explaining sound like air in the trans pump. How many miles on the truck. Have you ever changed the tranny filter? The other thing i mentiond was the torque converter. I have read on other sites that they are known to go bad. It has something to do with the vains inside. Does the truck shift alright or does the tranny slip and feel slugish? Also is the truck a 4x4? If it is try shifting the trans. case into hi and low and see if the sound goes away. Let me know what you find. I'll try to help you figure this if i can. Good luck!!