I opened my sunroof fully this morning and suddenly the car made a deep, buffeting wind noise similar to a subwoofer bass speaker fluttering. It happened only with all windows rolled-up and nearing a stop. WTF?

Car is a 2002 allroad 2.7T with Tiptronic...

 

pressure differential caused by air movement over the open sunroof. you can either try a deflector, crack your windows a smidge or try adjusting how far open your sunroof is.

Its normal, nothing to be too concerned about

 

You can reduce it by closing the sunroof a little or drown it out by cracking the rear windows, but otherwise it seems to occur around 40-45 and then goes away. Nothing else you can do.