Both of my transmissions that I had to replace started out whining in 2nd. Then proceded to 3rd and 5th (will not whine in 4th, direct drive). My last one (X-90) I ran for 15-20,000 miles after the 2nd gear whine started, before it gave out. Definately service it and drive it. If it is the bearing going out, no worries untill 5th starts whining.
Zig