You may be hearing the fuel pump located in the fuel tank priming to achieve start-up pressure. It makes a low frequency thumping when the drivers door is opened. The fuel tank is located below the 3rd row seat and the sound is a similar frequency to the rear seat motors. It is priming in anticipation of a start event. Curiously, it will do the same thing under seemingly random conditions when turned off and doors closed as well.
I was installing my brand new Hellwig Rear stabilizer bar yesterday and the pump primed up while I was underneath the tank installing the bar. The truck had been turned off and on the ramps for ~20 minutes at that point with all doors closed. I nearly experienced a boxer changing event.
