I posted this on the ST-10 topic:
If you put your ear near the speaker, when there is no music playing, the amp is in an "unknown" state.
Our amps actually do not use external PWM signal for modulation. Instead, we use the music to create a modulating PWM signal (I know, this is too complicated to explain). PWM modulation is fundamental to Class-D design.
Now, think about this for a second. If there is no music, what's going to happen to the amp? It is in an unknown state and usually you hear some noise.