I don't know if I could help - but its a try.
A high pitch whistling sound would mean some wind creeping in through the rubber seals that wraps around the door. If you say its the right rear pax door, do check also the right front door for a joint possibility. I assume nothing is protruding on the outside as that would also create a kinda of a wind-whistle effect at high speed.
Before we begin this exercise, do another visual inspection of all seals, gaskets, door mountings and frame to ensure that no gaps are present. Most prevailing are gaps within the rubber seals, or sometimes (rarely) gaps at the panels where the clip has fallen off). You might also want to wind down the windows and check the seal-felt that tightens the glass-window edges.
Proceeding with the assumption that it could be the rubber seal, close both the right hand doors, front and rear. Use a musking tape to tape all the edges of both doors.
Now drive at a stretch to the offending speed and listen. If the wind whistles yet again, it could be a door frame problem itself. If the whistling is absent, then peel of the tape stretch by stretch as you drive. The idea is to eliminate the possibilities of which door seal is letting through the wind.
Once you peeled off the offending tape and the whistle comes on again - you will know which spot and then able to concentrate on that area, perhaps a harden rubber seal.
Now, if the test was not fruitful, do a final salvage test by taking out a piece of cardboard. Place the piece of cardboard-paper sticking out of the door frame, and shut the door over it. See if you're able to slip out the cardboard with relative ease. If you can, then perhaps your doors have to be tighten inwards. If you can't, then its good.
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126 tailed by a 203, 129 leading the pack.