First, be sure your parking brake isn't on and that the transmission is in Lo.
This is the obvious stuff. After that? It is what it is. Some guys report the harmonics are amplified through the ROPS on some models and report success with foaming the ROPS to kill the transmission of the sound waves. Also, the nature of the HST is that if you push the pedal down hard, you aren't providing more energy, of course, but making the gearing even taller. It is a bit counter-intuitive, but forcing oneself of the habit of mashing the HST pedal when wanting "more" takes some time to break.
Yes, you'll get used to the noise, somewhat.