When my IS250 has been driving a while and everything is hot, from time to time when I put it in D or R and have all the music turned off, And i am holding the car stationary with the brakes, I can hear this strange rumble coming from the back of the car. Sounds like under the trunk. It's not an exhaust sound. Its very subtle and is impossible to record. It's just one of those annoying hums that if you know its there you can hear it.
As soon as I take it off gear and put on N or P, then it disappears. And also disappears if I come off the brakes and let the car move. Only happens when hot. Any ideas what this can be?