Our 2010 Prius developed a sound, one cold morning, when the engine started up as my wife backed the car out of the garage, it sounded like playing cards in the spokes of a bicycle wheel (ahh childhood memories). Since the car was due for it’s 20K check, I mentioned it to our friendly Toyota Service guy and he said he’d check. When I picked up the car later in the day I asked about the car he said everything was OK and they found the noise source. It was a mouse nest in the heater fan. The filter was replaced, the nest cleaned out, no problem. He has seen the problem before, because the mice are looking for warmth in the dead of winter. He suggested two things. (1) Scented Dryer Anti-Static Cloth, used to stop clothes clinging together, tie a couple off under the hood in various places, rodents cannot stand the odor, (2) Spray FABREZE into the engine compartment every so often, again, rodents cannot stand the smell. Seems to work.