I listened to the car for awhile and noticed a whining sound coming from the rear of car. It was very faint and sounded like a real slow squeak (not high in pitch) - kind of like somebody cleaning a glass window.
I thought it might be pressure escaping from the gas cap, so I opened it and released the pressure. Got back into the car and could still hear it. You can't hear it from the outside of the car; there's too much engine and exhaust noise.
Whether the car was in park or drive, you can still hear and the frequency didn't seem to change. I tried revving the engine in park, but cabin noise increases and you can't hear the squeak any more - I want to see how it changed with acceleration.
Anyway, I'm guessing this is the fuel pump crying, "Replace me!"...