Yes, it's the lift that seems to fade away as the unit warms up. When it is cold, it lifts quickly &amp; smartly. As it warms, it is slower, until, at full operating temp, it will not even lift the brush hog fully off the ground. -It lifts the front end some, but the back wheel stays on the ground. When the hydraulics are fully warmed up, it won't hold it off the ground long at all when shut down -That's after I help it lift in the first place..... 
I too, believe that I cavitated the pump when I first took it on that steep slope.... It was the first time I heard that whine, and that was when I noticed the hydraulics had gotten quite hot. It still gets pretty hot if I run it long, or AT HIGH RPM for any length of time. -Don't know the temp, but it's hot enough that I can't hold the dipstick for long. If I keep the RPM's at around 16-1700, I can brush hog for about 45 mins - 1 hr. without getting too hot. 1800- 2000 RPM, ant it doesn't take long.... The whining is present when it gets hot, and doesn't want to lift, but not as bad as when it first did it ( cavitation day ).