The aerodynamics causing the pressure difference of the air between the outside and the inside of the car.
If the airflow over the roof remains attached, i.e. laminar, then the buffeting noise will be much less. If the airflow is turbulent, noise levels will be enhanced.
This can be explained by complex 'Boundary Layer Theory'.
The buffeting can be quite bad on some cars where not enough attention has been paid to the positioning of the sunroof and to the height and orientation of the wind deflector.
So if the opening is too large, the pressure difference will be greater and the buffeting noise levels will increase.
Reduce the size of the opening by slightly closing the roof or reduce the pressure difference by dropping a window.
Can't be bothered with sunroofs myself.
Helmholtz resonator? An open roof acts like an organ pipe, exciting the acoustic resonance of the interior air volume - I think. Works with side windows too.
I thought about this too:
frequency of resonance = [c/(2Pi)] x [square root of s/(lv)]
c = speed of sound
s = cross section area of neck (sunroof opening)
l = length of neck
v = volume of car interior
The narrow neck is the sunroof opening and the inert volume is the cavity of the car. This causes an impedence mismatch between the air on the outside and that inside the car due to the narrow neck which is the sunroof.
Changing 's' would affect the freq of resonance. Closing the sunroof a bit will reduce 's' and lower the resonant freq of the system.
The mechanical analogy is the mass/spring system.
With a Helmholtz resonator, the coupling of the air on the outside and the air in the volume is very weak due to the narrow neck. This produces an impedence mismatch. Additionally, the Helmholz resonator is an acoustic filter (Quincke filter) and cannot produce any sound.
In the case of the organ pipe, the coupling of the air inside the pipe and the outside is good. Hence the organ pipe produces sound.
It is an interesting bit of Physics and a variety of theories may be used to explain this phenomenon.
