Radiator fan ?/help

I have a 2007 Prius w/78K miles. This afternoon I had something happen that I don't recall happening before, and I am wondering if it's just something I've missed that is normal -- or if it is a sign of impending trouble.

I'd driven about 30 minutes at speeds ranging up to about 45 mph; ambient temperature was about 70 degrees. I pulled into my garage and spent a few minutes listening to something interesting on the radio without shutting down the engine.

I noticed a rhythmic noise or vibration from under the hood that I didn't recognize. It would occur for 10-15 seconds, then stop for about the same period of time, then start over. This kept up for three or four minutes, and I decided to pop the hood to see what I could find out.

The noise seemed to be the radiator fan cycling on and off -- I say this because I put my hand on the top of the fan and felt it vibrating in synch with what I'd been hearing from inside the car. I don't remember ever hearing this; perhaps I just wasn't listening.

During a point when the fan was running, I turned off the car and the engine stopped.

This seemed odd to me -- I've owned other Toyotas with similar electric fans and those fans would stay on after I turned off the ignition if the engine needed to be cooled.

Do I need to start worrying, or am I OK?

 

A/C was on. I guess that explains things. Thanks, Patrick!