The light weight means they don't internally dampen the sound much, as they just don't have the mass of material to soak up the vibrations.
The amount of noise mainly depends on the torque the motor is creating. So high power at low speed is noisy.
I have been pretty happy with the noise levels when accelerating at 500w, at 1000w I don't find it as stealth. Above that and you will draw attention.
At speed I just hear wind noise, and can only just hear the motor.
Here is an example of the sound a friction drive bike makes accelerating.
36sec to ~42 secs the bike is accelerating, then from then until about 45 seconds it is just cruising at speed.http://www.youtube.com/watch?v=IotizzHKYdI#t=32