I had 5 different 770s in hand and yes the humming is there. Some are a bit more loud than others but none is silent and the difference is not big. The only way how to make it silent is to set brightness to maximum. I'd guess it is design fault/feature. Some PalmOS PDAs have noisy screens too. N800 is silent.

Also on lowest brightness screen sometimes blinks. More when on charger or when wi-fi is enabled. Again all units I've seen do it, some more some less. My N800 does it too.