2005 Sprinter 311CDi whinning nose when not under load

[SprinterSprintShift]

Hi all, i was wondering if anyone has had this problem:

When accelerating up a hill the whinning stops, if i stop accelerating the noise comes back also when accelerating downhill over 60 it makes the same noise

Any help would be much appreciated

Regards

Btw its the van has a sprintshift box (semi-automatic)