Help! We have a 2004 F150 FX4 that has an issue that we just can't figure out. When you press on the gas to accelerate from a stop, it goes fine from 1st gear but as soon as it goes to the next gear or any gear, there is a "hollow" sound in the engine and it barely will accelerate. If you let your foot off the gas and put it back down it gets up and goes. The ford garage says it has no codes showing up. We have changed the cam phasers, fuel filter and throttle position sensor so far. It actually seems a bit worse since we changed the throttle position sensor. Does any one have ANY other ideas as to what this is? It is more of an annoyance right now, however, on the highway when you are trying to get up to speed or speed up, its a problem.