I wonder if what you are hearing at less than full throttle is the sound of the bypass valve cycling open and closed rapidly enough to be described as a frequency or squeal in response to pump volume being only slightly more than what is needed by the drive motors for the speed you are going, with the result that pressure is forming a sine wave above and below bypass pressure, and hence the bypass valve is opening, then pressure drops, it closes, pressure rises, it opens again etc. etc. That might also be accompanied by whistling of the hyd fluid going through a minimal opening in the bypass valve. If you increase the motor speed, then the bypass might be open nearly all the time, and/or cycling at a higher frequency, and/or opening the bypass a little farther eliminating the whistle, thereby collectively producing no or a different sound. I would expect the least noise to be present when "demand" (fluid potentially sent to the drive motors) is greater than "supply" (pump volume) with the result that the bypass valve never opens because under those conditions excess pressure cannot be generated.
Just a thought.