[iainb]

It's called wind buffeting. Almost every car will do the same thing in the right circumstance. You open up a single air "intake" without giving the air anywhere to flow, it pulsates/buffets.

Try rolling down a window or two about an inch when you have the sunroof open. Should alleviate the problem.

This.

In our Jetta the sunroof has a "comfort" setting- if you turn the knob to open the roof it will open all but the last 2 inches. If you open the last two inches (by holding the switch) you will get horrible wind buffeting unless you open two of the windows.