Excellent discussion. My line of work was mechanical for the last 20 years. And when on my M413X or even more so the LS, my ear protection is in. Unpleasant frequencies indeed. Tires your head.SNIP
The hydro growl, or more correctly whine, is courtesy of the pulse frequencies of a positive displacement pump as each chamber of the pump unloads into the system. As the pressure rises, the whine gets louder. Anyone who has worked in a hydraulic pump room knows this in their bones. Hearing protection is a very desirable piece of equipment.
A little anecdote about heat generation. On the M413X I literally sit on the hydraulic pump. When I bought the unit, the AC was inop. That was a high priority repair, had to find specialty 134a fittings pronto. The heater is bypassed. I wondered why anybody would do that ... until a few weeks back, working in the 40s and realizing that the hydraulic pump under my seat provides all the warmth one needs after about 20 minutes of grading. The wheel drive motors are also hydraulic so all motive power requires that big pump, it's right on the 4BT crank.