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Its not surprising that there is more noise when a window is open, but it is interesting that you can sense a difference between the 2 cars.
I would suspect that the difference is due to the shape of the respective cars, and how that affects the airflow over/around them.
The Savanna is a comparatively low-slung 'normal-shaped' car - if you look at it head on, its frontal area (the size of the hole of air it has to displace whilst moving), is a lot smaller than an Espace (at least by 50%).
What this translates into, is that the air flowing around the Espace is under a greater amount of pressure, and as air has a natural tendency to flow from high to low pressure - then there is a higher 'force' driving the air into the cabin.
Cabin shapes may influence the acoustics (i.e. if you have the rear seats removed, it may sound different compared to when they are fully occupied), but the over-riding principle is the same.
Hope that helps.
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