Loud Noise (from radiator fan?)

A friend recently drove her 2008 Prius here from her home about 300 miles away. When she arrived, I noticed that the car was making a loud noise. It sounded as if the radiator fan was running at a high speed. I didn't remove the shroud to expose the fan but that certainly seemed to be where the sound was coming from. We both thought that it might have happened because she had been driving most of the distance at speeds of around 80 mph on a hot day.

However, when the car was started the next day after being off for about 16 hours, the same loud noise started as soon as she pushed the power button, several seconds before the gasoline engine fired up. Since nothing else seemed to be wrong and there were no warning lights showing, she decided to drive it home.

She just sent me an e-mail saying that the dealer she took it to said there was no problem. I know from the almost five years I've been driving my 2007 Prius that this sound is definitely not normal. In fact, if all Prii were this noisy, there would be no complaints about them being hazards to pedestrians who can't hear them coming. Can anyone suggest what the cause might be?

 

Thanks, Patrick. I'll pass this on to her.