Puzzled in AZ

Well at least I’m not alone. Our 2016 RX350 has this sound coming from the rear of the car after it has sat in the garage for three to four hours after it has been operated. At first I thought it was the hatch motor. But cycling the hatch didn’t remedy the noise. I had to start the engine for a minute or two and then shut it off for the humming noise to stop. It’s not a frequent issue I don’t think, but it concerns me that it is still doing it. Does anyone have a technical explanation for this phenomenon? I’d rather have some back up from other owners before trying to explain this to our service tech.