Giant G2700HD+, 50HP Kubota, 250 hrs, hyd. oil good & full. Never heard a hydrostatic drive moan like this. Cavitation?
I'd check oil level first. Then the filter and hoses. Maybe a hose is collapsing or restricting flow?? How many hours and when was the filter last changed? If the whine is new I wouldn't run it much until you figure out and fix the problem.
Like I stated, oil level is on the full mark on the dipstick, original filter. Hoses all check good visually. I bought the machine used, and it just turned 275 hours. It has always made the howling noise but seems to operate fine. The howling pitch gets higher the faster you travel.
Did it always whine? Some hydrostatic trans. whine all the time and are worse under load. Higher speed puts it under more load. It might be normal. I had an older skid steer with a loud whine but nothing was wrong with it.