Whistling noise from the rear

we have 99 RX300.
has 116K miles.
about 2 years ago, started developing a whistling noise somewhere from the rear. when i seat in the driver seat, it's coming from behind my right shoulder, from the back, and about my ear level. i am 6'5''. basically, sounds like upper part of tailgate.
it comes up only at high speeds. sometimes it's 55, always 65 and up. starts with slight hiss then turns into what i can describe as low pitch flute sound. whoooooooooooooo. it is speed relevant, pitch goes higher as speed increases.
it is absolutely annoying.
so far, i had additional seals added to the entire tailgate. rear roof rack removed. had entire tailgate taped around so that no air gets in. tried create positive pressure inside and sprayed soapy water on all doors and tailgate. nothing helps.
i asked my sons several times to locate the sound origin, while driving, they can't. it's somewhere in the back is there response. i tried it myself, and still can't pinpoint where it's coming from.
it's quite loud, actually.
yes, i did have rear diff fluid changed at 65K. it does not sound metallic or crunchy, like bearings. i have no idea.
anyone had this?