Slow F350 Dually

ford
f350

#1

I have a Ford F350 Dually with a 7.3L diesel. Under cold conditions, especially if I don’t plug in the block heater in the winter, It is slow to accelerate and will not go over 25 mph. If i shift to neutral the engine revs normally. when I put it in gear and press the accelerator to floor, it feels like it is being held back, there is shooshing sound under the passenger seat. After it warms up it is accelerates normally. What’s up?