Ford F250 diesel engine noises



I have a 2004 F250 pickup with a 6.3 liter engine. Lately when I’m driving up hills more that 45mph and let off on the accelerator I hear a 3-4 second back and forth swishing sound coming from the engine compartment and feel a vibration. It only occurs when the engine is going up a hill and usually just if I let off on the accelerator. Otherwise, the pickup seems to run well. It seems to me that this has been a problem since buying the truck new but it’s gotten more noticeable recently.