Loud Fan Noise on Long Idles

Highly UNlikely that there is a problem.
In "normal" operation, you may have never heard the fan run before.

 

The car will periodically run the ICE to recharge the HV batteries when the electrically driven A/C draws down the HV supply. The A/C fan runs intermittently, especially when the HVAC system runs in closed cabin mode, not having to pre-condition hotter outside air. This is what you and the kiddo are hearing. Nothing to be concerned about.

 

While the car may run properly there are concerns with long idles in a gen3 hybrid. First, I have measured significant thermal cycling of the engine coolant and by definition the engine under these conditions. As much as 50f in just ten minutes. Given this model has head gasket problems, thermal cycling is likely to aggravate that potentially big repair.

The ac and other systems are running off the high voltage battery until the battery is somewhat discharged. At that point the engine starts in order to charge the hv battery. This cycle will happen several times an hour because the system only partially recharges the hv battery at "idle". Less charge means the hv battery discharges quicker than the initial cycle. When driving the system maintains a more fully charged battery for longer periods.

Under some conditions there could be a small risk of CO poisoning from exhaust gasses entering the ac air intake.

Overall its not a good idea, especially because of the thermal cycling.

 

how many miles on her?

 

You may want to make sure the kiddo's blanket isn't covering the battery cooling fan intake vent. Rear seat, below the cushions. It probably time to take it apart and clean it out. If that vent and fan isn't functioning properly, you'll shorten the life of your traction battery pack.

Hope this helps..

 

i don't see any problem letting the car sit in ready, people camp in them overnight with no known ill effects.
prius was designed to continually turn the engine on and off to charge the battery, and the battery to run the accessories.
i agree with rj above, it would be good if you can find a mech who can do a thorough cleaning of your egr circuit, and perhaps add an oil catch can, to help protect your head gasket.