1. having window open too much makes that horrible helicopter noise (I won’t use the technical term). Panoramic roof is fine
That’s because there will be a lip ahead of the panoramic roof which pops up when opened, and disrupts the fast moving laminar airflow by directing it up and away from the panoramic roof opening.
The buffeting is caused when the air flow is smooth and attached - that is to say that it’s “tight” against the car when it encounters an opening, and is moving quickly and therefore low pressure.
The air pressure differential between the inside and outside is significant and abrupt between the static air inside and the fast moving laminar flow outside, without a turbulent buffer between the two.
This causes a resonant vibration which wouldn’t happen if the smooth laminar flow is disturbed and forced to separate from the car, creating space for a turbulent transition for the pressure drop.
It wasn’t a problem years ago as cars weren’t aerodynamically efficient and so there wasn’t much - if any - laminar flow along the side of the body, it was mostly separated and hence inefficient.
The turbulence increases aerodynamic drag and so modern cars are designed to maximise laminar atirflow for aerodynamic efficiency, and therefore fuel efficiency, so it would affect modern cars much more.
However modern wing mirrors are designed to separate air for the front windows but there’s often sufficient laminar flow reattached to the surface for buffering to occur with open rear windows on most cars.
Very aerodynamically efficient cars - often EVs are the most efficient - sacrifice the lack of benefit of reduced buffeting for improved efficiency and so wing mirrors are designed to minimise disruption to the laminar flow.
Modern cars - and especially aerodynamically efficient cars like your EQE - have very tightly sealed panels and relatively little air leaks from the cabin, which exaggerates the resonant effect.
This is why opening another window - sunroof or panoramic roof - even very slightly stops the resonant vibration as it allows air to leak from the cabin to the outside. Popping you roof on tilt is ideal.
Also opening the partially open window further usually eliminates buffeting because it disrupts the airflow to cause enough turbulence to create a much larger transition for the pressure drop.
