You may want to compare it to another truck at idle. diesel has a rattle sound that is - i guess you could say normal or natural or something.
also when cresting a hill or rapid desceleration after being under load you may hear your wastegate. it releases the pressure from the turbo in advance of the engine, when it is necessary. the sound from this usually gets more pronounced after a mod- do to a much higher psi
as an experiment try accellerating rapidly, and at the upper end before it flattens out try decelerating rapidly. at just that moment when you press the brake, ( no need to try and stop on any change, just decelerate.) pay attention to the sounds you hear and try to determine if the noise at the moment your rpm drops is the one you are asking about.
if so it is possible that you are hearing standard engine sounds and the wastegate allowing you a controlled deceleration.
Don't worry too much yet - diesels are loud, probably part of why you bought one.