Mhart, yes that is correct, mine is making the exact same sound, drives me crazy. It use to be during the winter only but over the last few years, the sound comes on every time when I attempt travel speeds over 1500 rpm. the sound stops only once I take my foot off of the hydrostatic pedal and the tractor slows down. At first I thought the sound was due to oil starvation at the hydrostatic pump, sounded like an extreme cases of pump cavitation. The sound would quit once pump demand was reduced, when the tractor was slowed down enough or fully stopped, but now, I'm not so sure that it is pump related at all. PLUS I've done several hydraulic oil and filter filter changes, including suction screen cleaning, cut the filters apart and they never show metal fragments or contamination. I. think this is somehow caused by shaft rotation.
I was going to take the front driveshaft off at some point and test drive the tractor at 2100 rpm. Haven't done that yet. Let's hope some one else on this TBN post can help figure this out.
Dan