My w56 with duals started making a whine in the 2nd and 3rd after i changed the gear oil over the weekend.....it is fine in 1st and 4th.
The tranny was always a bit noisey in 3rd and 5th but more of a growel then a whine but after changing the oil in the tranny, crawl box and t-case it started making the whine in 2nd and 3rd.....sometimes it goes away tho.
I think there is enough gear oil in the tranny, i put two 1 litre jugs in although i didnt crack the fill plug since i have a flat belly the ehaust is right in the way.