Sudden Gearbox Whine when towing

Hoping someone can shed some light...

I was towing a car on a trailer tonight (about 2.5 tonnes) in my 200tdi up a fairy steep hill when I started to get a whine from the gearbox. It would only appear under power and mainly in 3rd - 5th. I stopped to let everything cool down and the whine almost went away completely. Once I’d dropped the trailer off the whine disappeared. I’m going to check my oil levels and probably change the oil in my transfer box and gearbox tomorrow and see what I find but I just wondered if anyone had experienced something similar?

Oil check/change is the way to go.

My TD5 did this at motorway speeds after a remap.
A loud whine at certain speeds.
The remap was removed and the whine disappeared.
I changed the oil in the gearbox/transfer box/diffs.
Some time later l had it remapped again, that was over two years ago and the whine has never re appeared