Carguy0035 is correct. When you turn the Prius on the stored hot coolant is pumped from the thermos to the intake manifold. This warms unburnt gasses left in the intake manifold so that they will burn cleaner when the engine starts. This lasts for seven seconds, then the engine starts to warm the catalytic converters. When you power off the Prius, hot coolant from the engine is pumped into the thermos.
Notes:
1. If the noise is exceptionally loud the pump may need replacing.
2. The Prius checks the temperature of the coolant occasionally, both when the engine is running and when the Prius is off. It will move coolant from the thermos to the engine at various times when the engine is running. If the coolant temperature in the thermos gets too cold (when the Prius is off) it will skip the transfer on startup.
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What's that noise my Prius makes when I turn it off?
Linear Mode
