I have a 65 stingray coupe, 327... 4-speed. I have owned it for four years and from day one it has had a light whinning noise when going thru the first three gears, but running quite in 4th. I don't think it's a M-22, but a M-21 trans.
This past week I changed out the stock exhaust pipes and mufflers to an original style off-road chambered exhaust system. Now even with the increased "growl" of the new pipes, the whine seems MORE noticeable!
Question..... Could the whinning be coming from the drive shaft rather than the trans??? I have 336 posi rear end gearing, stock.
Thanks for any info guys!!!

Generally, the whininng noise is from worn syncros. You'll notice it when slipping into gears.
The hollow drive shaft will resonate the whine. Bad u-joints in the drive line may squeek, but not whine. Rear gears that whine, will have a constant whine, no matter what gear you're in.
Hope this helps. :)

Thanks for the info C3.... I figured my trans was getting worse. Now I'll have to get to work and have it rebuilt!

I think your problem is more likely due to a bad input or output bearing. If I were to guess, based on your info, I would look at the input first.