Faaint Fan(?) Noise When Prius Parked

After I've parked our 2010 Prius in the garage and many hours (like 5+) later re-enter the garage for something, I sometimes hear a faint noise (like a small fan might make) coming from under the rear of the car. It is especially easy to hear if I put my ear down by either rear wheel. It's not hot and the car has been parked for many hours.

The sound does stop but I don't know how long it goes on. Typically I've heard it in the evening and it is quiet the next morning.

Suggestions as to what that might be?

-- Larry