Hi,
I would check the fluid level. Is there fluid in the sight glass when on level ground? Did you pull the dipstick and verify the level? (it's below the range selector lever, you'll need a rod to hook the dipstick and pull it out.)
Hydrostats make noise if the pump picks up air and they don't have any power at that point either. (The guy next door found this out on his Cub lawn tractor last year)
If your backing up a hill and the fluid level is low you could be uncovering the pickup for the pump which I believe is to the rear on the 2x20 series. (not 100% on that though)
Tom
You are going to have to show me a picture of the dipstick, all I have is a fill cap and there is nothing in it, I reached down in there as far as it goes.
As for the sight glass, it looks red, but I can't tell if it is full empty, I assume full since the fluid is red.