Max, I think the buzzing could be caused by the PWM frequency. I am only just starting my learning curve on Loksound, so this may be off base but here goes anyway... 
Out of the box, the manual reads like Loksound uses a variable PWM frequency for fine resolution motor control when back EMF is on and motor under some load. This may be why you don't hear it on the test rig with very little load, and why the much lower current-draw Atlas motors don't "drag the PWM frequency down" as far to get smooth start and slow running . It says with some motors this will cause a buzz. It is possible to adjust the PWM frequency to fixed using CV124. Thus should keep it out of audible frequency range and hopefully keep most of the back EMF control. See page 48 of the Loksound V4 manual. Seems bit 4 can be set to change the PWM frequency.
It may reduce torque or reduce slow running quality a bit but definitely worth a try imho. I haven't tried this yet myself as I only have one Loksound in a Yoder 44-Tonner and that ran great out of the box, no buzzing. But that uses tiny can motors that draw about 1/10th of the current the Pittmans do. 
Interested to see what you find!
