My van is singing, like a seatbelt ding. It dings in sets of 5. It will do that 3 times, then not again for a while. Any ideas

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Its the same ding, as when I leave the light on or the keys are in the ignition and the door is open. It does it when I start the vehicle, dings 5 times, stops, dings 5 times, stops, then repeat. Then stops. Happens about every 30 min or so. While I am driving.