[dirtyd0g]

On early model units the impeller blades liked to come loose. The way to see if that is the issue is get on and off the gas at about 20mph in 2nd. They will give a chingy/changy noise as you get on and off the throttle. In the later years (03+) the fins are furnace brazed. That eliminated the problem. The next issue inside the converter that likes to cause that problem is the rivets on the turbine hub. Again in the later years ford revised these to prevent the problem. Early models were small head and hollow. The late ones have a huge head on them and are solid. Your truck should not have any of those issues but if it is a 4.6 (or 4.2) in 03 it may not have been updated yet. 5.4's got updated way before that.
Alan

The reason I say 03 is because it is possible to have an 03 trans in an 04 truck.