Every once in a while I go into my garage at night and hear a sound similar to air being let out of tires coming from the rear of the car. Of course, the tires never deflate. I would also occasionally hear this on my prior car, a 2006 Corolla. Any idea what this is? Thanks
My guess would be it is a sound from the fuel vapor management system.
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+1
The Owner’s Manual for my friend’s Rav-4 mentions the “normal” noise from the periodic self-test of the vehicle’s Evaporative Emissions system. That self-test is run after the vehicle has been idle for several hours.
I would be surprised if the Prius Owner’s Manual didn’t mention this situation.
Has the OP perused that manual?
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Surely you jest. 
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