Hi bollermann!
It is not the dashboard.
I removed a lot of it like you say, and went on a ride with a passenger.
We held our hand on everything we could touch and it is still rattling.
Thus i think, and also when you listens close, it is deeper near the firewall.
Seeing that the noise dissappears when i swerve on the highway, i think it is somewhere in the subframe-steering-suspension.
ah, ok.
Nice riddle to solve 
Yes, maybe something in the area of steering/suspension.
But maybe through the little winding of the frame the millimetre or what of space that makes the difference between rattling or been still is the cause. Hard to try-and-error-out.
For with the suspension I only got the idea of the metal shielding of the brakes making a noise. Because they are fixed in relation to the frame and a wheel-bearing/wheel with a little too much wear moves slightly in relation to the fixed metal-shielding while steering to lef/right.
But then, you should be able to hear it from the outside... so not very likely.
But anyway, now I´m done with speculating, no more ideas 
The car is going (again) to the dealer in 3 weeks or so.
Maintenance and the rattle… i wil update when i know more. Thanks for helping out!
Brought the car to the dealership for maintenance and the rattle.
The chef said that i wont get the car back if it’s repaired properly.
I asked/told him what do we do if it’s not fixable.
He said that we will not discuss that now and he is sure that this can be fixed.
Let’s hope so, because i love the car! Update soon i hope.
Die ARMLEHNE !!! 
... wenn sie nach vorne geschoben ist. 