diff noises

hi there can anyone help as i am new to owning a td5 disco 6 weeks ago had full service done all oils changed now on over run or doing 40/50 and dipping the clutch you get a loud gear noise when load re applied ie throttle back on noise go's could it be wrong oil or is there an additive someone could have put in as old oil seemed alot thicker.

My Disco 300 makes a whirring noise when I bring the clutch up sometimes. Most likely culprit is I suspect the clutch release bearing.
If it sounds unpleasantly metallic or grinding, I would suspect a gearbox problem. See if you can find out what oils they put in the gearbox.
Ashcroft transmissions are as good as anyone for specialist advise on Landy gearboxes.