I believe it can vary even in the same manufacturer line, same models. I had an LS mt125 SCUT.... the HST whined loudly all the time. My neighbor bought a 125, it was about half as loud as mine.
We both "upgraded".... me to a shuttle shift gear LS XG3025. He went with the new MT225-S, again HST. His is absolutely barely audible....all you hear is kinda like when the hydraulics bypass....
The gear lash is far louder on mine, especially in the lower gears. Definitely not as loud as my previous HST version.