I've not seen any posts on this, but every benz I've owned from the first 76' 300D thru a 92 TD 4matic had some wierd harmonic vibration with the roof open and no window cracked down a bit. My first guess is the sliding headliner has come loose in some way, and while it retracts properly when the roof is opened (right?); wind causes it to flap around.
Next guess is the roof itself not staying tightly in the track when fully open.
In either case, it takes a combination of speed and air pressure to set the vibration up.
These are adjustable issues, and the appropriate shop manual if you are a DIYer should give you the answers.