What causes the cooling fans to vibrate and hum ?

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while being stuck in traffic and at a stand still my van all of a sudden starting a loud humming and vibration sound. at first I thought something happened to the air conditioner so I turned it off and turned of the ignition. I waited a few minutes and started my van up with out the a/c running, after idling for a little bit the loud hum and vibration started again. well by that time traffic started moving so I started driving and after a few minutes the hum and vibration stopped until I was at an idle again then it started again when the radiator fans kicked on, well traffic started moving again and after a few minutes of driving it stopped making that vibration until the next stop and idle.