Whistling Noise From Back Seat

[LT2211]

My ES350 has developed a loud whsitling type wind noise from some where in the rear of the vehicle. A very high pitched whistle that increases in loudness with more speed. From the front seat, one might think the noise coming from the back doors. When seated in the back seat, it sounds like it is yet in back of you, rear window, trunk? I haven't seen any similar complaints. The noise is loud enough with the radio off to interfere with normal conversation and is annoying.