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Sounds like a worn diff, where the teeth are worn and when on the overrun the wheels drive the transmission there is a "howl" / whine. Although a mech enginer at work tells me helical gears are only designed to run in 1 direction so in "reverse" they may well make a noise.
I had mine swapped for about £100 by Rally Colin, Thames Valley Motorsport and it's far quieter now. I also ad 2 piece propshaft swap[ped for a 1 piece one as I also had clicking noises which may have been prop U/Js.