The only thing hot enough to burn anything would be the exhaust manifold and pipe. Look around the exhaust manifold on the engine to see if there are any hoses or wires, wiring harnesses close to the exhaust.
Check to see if one of the pulleys that the belt turns has a bearing going out. This could overheat the belt causing a burning rubber smell. Also running over a plastic shopping bag can cause the smell when the bag sticks to the hot exhaust under the vehicle.
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