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There are 2 sets of sounds you may hear when you just powered down. One is related to the cooling system, which Firepac mentioned. First a valve changes position to connect the thermos to the engine cooling system. Then you hear an electric pump pull the hot coolant into the engine, exchanging it with the cooler coolant originally in the thermos.

Later, you may hear a hissing sound, followed by grumbling sound. The hissing is the brake system releasing brake pressure from the wheel cylinders (and probably the accumulator), then the accumulator pump restores brake pressure. Not sure of the exact purpose of this process, could be to try and purge any air in the system, or possibly a system check.
 
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