Buzzing/vibrating noise coming from center dash area while truck is off?

I am renting a U-Haul pickup truck currently and I've noticed a vibrating/buzzing sound similar to the way a cell phone vibrates coming from what sounds like under the dash near the center. It's not a constant sound and I only hear it every so often, and I only hear it when the truck is off. I don't know what it is and would like to know if it's normal or an indication of some kind of problem.