Weird door noice/rattle

[asmodeus812]

Hi all, I have been experiencing a very weird, impossible to pinpoint door rattle. It is coming from the right/passenger door. I have removed, for what it seems, at least 10 times the door panel to try to pinpoint the rattle. The sound is very metal like. It is 100% reproducible in a very narrow speed window - when the car is moving between 90 and 115 km/h over a mildly wavy road (not hard or bumpy, nor filled with potholes etc). It produces a very steady clunking noise which is monotone, sometimes getting slightly louder sometimes getting softer. The rattle is perfectly reproducible and predictable.

The location of the rattle is behind the door panel itself, and between the two metal layers of door frame where the window inserts into the door. The picture below is taken from the internet, used for reference, i have tried to circle the suspected location of the rattle.

What I have tried so far that did not affect the noise at all
- taken out the door panel and drove the car around without it
- taken out the window cover on the inside (the long black triangular piece of plastic that is decorative and hides the door frame on the top from the inside)
- removed the plastic cover around the door latch (which is slightly overlapped/covered by the door panel)
- tightened all bolts that are visible and accessible on the door frame itself (behind the door panel/cover)
- tightened the window regulator/lifter (put the window 80% down, and made sure the exposed bolt is tight)
- tried wrapping duct tape around the door striker
- tried moving the door striker up/down and/or forward/backward
- tried completely removing the door striker (drove like that while using another person for assistance)
- made sure no cables or connectors are loose inside the door frame itself (and hitting the door from the inside)

Things I noticed along the way - the other day while messing around with the window regulator. I tried, while the window was down (at about 90%), pushing one of the scissor legs (that lifts the window up/down) in and out. The plastic white guide that is in the channel attached to the one of the scissors legs actually seemed very loose, and made quite strong noise/rattle. But when I put the window up completely and tried to reproduce that, it was rock solid and steady, not the slightest movement. I was not able to reproduce it by applying strong force up/down or forward/backward.