Recently my 04 6.0 has been "coughing" occasionally. Mostly around 50mph,trying to accelerate to road speed. It sounds like something is "venting" down low under the engine. Sounds just like the old plastic panel shaking under the front of a car. The engine misses when it coughs, but if I let off the accelerator it's okay. This problem doesn't happen all the time, doesn't seem to matter if the engine is hot or cold, or if I'm towing or not. It will go away after about a minute or so, but will come back again an hour or so later.

2004 6.0 xcab 2WD auto, (stock tune)