The PHEV is relatively quiet unless pushed hard. The engine gets loud and buzzy at higher RPM, but is quiet for normal cruising. Loud and buzzy isn't very technical, but the motor becomes like a typical high strung 4 cyl if pushed hard. Much like the Volt if you floor the go pedal in charge sustaining mode.
It isn't silky smooth revving like a BMW inline 6 that can rev up to redline with minimal vibration. Inline 6 engines are naturally balanced and don't have the vibrations that a 4 cyl has. That being said at lower RPM you can't even tell the Clarity PHEV is running, which is where it runs most of the time. I have to put up the energy gauge to know the engine is running most of the time, or look for the EV text on the cluster to disappear.
Accelerating I hear drivetrain or motor whine at times, but I think only because everything else is so quiet. Overall, it is definitely a step up from the Volt Gen 1 in terms of sound refinement.