What's causing a steady, high pitched sound that appears to be coming from back

What is your question?

I have a 2014 Lexus GX460 with 64,000 miles. I hear a low volume, high pitch sound that appears to be coming from the backseat area of the car. It's constant, doesn't ever get louder or softer. And when I shut off the car, the sound continues for about another 5 seconds or so. Any thoughts as to what can be causing this?

What makes this problem better or worse?

it's always the same. I just purchased the car and the temps have been consistently hot (Arizona)

How long has your 2014 Lexus GX460 had this problem?

I noticed it after purchasing the car, about six weeks ago.