Even on manual CPAP with no EPR, and no apnea to generate "central detetion" pressure pulses, the PRS1 does seem to sort of "pulse" on inhale and exhale. I notice it with a nasal mask as it sort of pumps up and down as I breathe.
I presume it's something to do with the machine adjusting the fan speed up and down trying to keep mask pressure constant as I inhale and exhale.
My S9 machine doesn't seem to do this to the same extend. I know ResMed claims the S9 motor responds more quickly and accurately to regulate pressure. The S9 just feels a lot smoother to me.