Slowburn

Hey folks, Having an issue with my '15 Double cab 4x4. I am getting a very strong odor, it almost smells like propane? I'm only noticing it when the truck is at full temp, and only when I come to a stop. the smell isn't coming from the vents, so I've concluded the A/C isn't the issue. I only smell it for less than 30 seconds, so by the time I get stopped and walk around the truck, its gone. Any Ideas? I'm at a complete loss.