Diff sound?

Hi all,
I have an 05 Impreza rs2.5 wagon, and when travelling at around 60mph in fifth and then letting the accelerator go there is a slight whining sound when the car is slowing down to about 58mph. it only happens at an exact speed around 60mph and not at any other speed and also only happens in fifth and second gear (only slight whining sound in second gear when deaccelerating with revs at around 4-5k).
noise definitely comes from front. is this the differential? and is it something that should concern me?

thanks a lot