The Prius That Shook.. - VFAQ Prius Site

crankshaft, resonance vibration occurs just like when an
ordinary car starts its engine. Resonance can be minimized if
engine rotation is increased quickly. Instead of using
combustion inside the cylinders to start, the electric motor
can be used. When the engine RPM reaches a certain level,
the electronically controlled fuel injection system (EFI)
injects fuel and the engine is started.
Vibration is most strongly felt while the car is stopped
and idling at a traffic light, for example. By completely
stopping the engine when the car is stopped, Prius can
best show off its characteristic quality. By designing the
engine to stop when the car is not moving, unless the air
conditioning is being used and by starting the engine only
after the RPM reaches above the resonance frequency range,
vibration was made less noticeable.
However, stopping the engine during idling phases
created a new problem. While noise in an ordinary car is not
noticeable due to constant idling and acceleration noise, it
becomes a noticeable problem in the case of Prius. This small
amount of noise due to relay switch operations was
minimized through increased use of insulation material.
When noise is measured inside Prius, it is far less than in
Corolla, but many, people are mislead into thinking that Prius
is just as noisy as Corolla because of preconceived
impressions.
317