I just took delivery of a brand new HD 44. Same exact sound was coming from mine...only when the PTO is engaged. It's coming from the flip up deck plate. At least thats what was happing with mine. I notice that you have your feet all the way forward off the plate while the PTO is engaged. Try engaging the PTO and move your feet back to the middle of the plate and put pressure down with your feet and see of the noise goes away. When you lift up the deck plate in the rear there is the round bar that goes across and underneath the plate. On either side of the bar you will see where the plate has been rubbing on the bar. It's a small area on both the left and right side of the bar. I seen the rub marks in the vid on yours as well. The fix was easy on mine. I'll try and post some photos on the Lawn site you have the thred going on. Hope this helps.
strick
strick