Help identifying noise (on 2013) - possibly under the hood

Yep-Retired said:

So, here's a twist. It was 59 degrees today. I drove my Expedition for a while to run errands in the bigger town, 25 miles away. When I got there and stopped at the first traffic light the sound started up. I turned off the radio to hear it better & sound kept going until I took my foot off brake. It happened a couple other times during idle. I parked in my brother's driveway, took key out of ignition & got out... we sat outside and visited for 30 minutes when suddenly the vehicle made the noise on its own. I opened the door to pop hood & sound stopped. He said it was the radiator fans. So I started the vehicle & let it run. The fans never came on and there was no noise. So, is it normal for the fans to kick on 30 minutes after the motor is turned off in what is now 52 degree weather? If so, fine... if not then I need to use my warranty before it expires this summer!

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