Why do I have wind noise in the dash area of my 1997 BMW 530 sounds like defrost blower once you get to 30 mph?

Look for loose plastic trim around the exterior edge of you windshield. For years, I would get a wind rattle at 50 mph whenever I was next to a big rig. The noise came straight through the center vent. Something about the physics of aerodynamics would lift the plastic trim at the base of the windshield. After a 50 cent plastic rivet, the wind rattle was gone. Another thing to check are the rubber jack points under the sills. if one or more of these are missing the wind will enter the body at this point and appear as wind noise at the windscreen. I had this problem and confirmed this by using tape to cover over the hole and went for a spin down the motorway. that's the cheap fix, the rubber mounts from BMW were a bit too expensive for me.