Sounds like a Kazoo in forward and reverse

[jinman]

Does your "kazoo" change pitch if you use different ranges? Sometimes at cold temperatures, you can get slight cavitation that sets up a vibration. Mechanical vibrations can sound like squeals, rattles, or the kazoo sound. Unfortunately, you can't go feeling around on the transmission or plumbing when you are driving the tractor. I'd just look for a loose metal part that could vibrate and cause the noise until the transmission smooths out as it goes up in temperature. Since the noise happens only when you drive the tractor, it's probably not the HST pump because that operates in neutral as well as in gear. It might be the HST motor or gear drivetrain. I think you can rule out the charge pump because that runs anytime the tractor's engine is running.