Annoying High Speed Dash Noise

Hi,

My 2000 Cavalier has an annoying noise coming from the dash at highway speeds. It doesn't happen all the time, and comes and goes. I don't know quite how to describe it, but its a sort of high-frequency rattle. It actually sounds somewhat like a table saw (as it's cutting).

The sound seems to emanate from the area of the windshield defroster outlet. It's independent of whether the climate control is on or off or what setting. It does seem to lessen if I'm behind a large vehicle so that there's not as much air moving past the car. Also, I noticed that the piece of plastic that goes across the top of the dash next to the windshield, presumably covering fasteners that hold the dash in place, has developed a crack.

Ideas, anyone? Is this a rattle between the dash and the body? Or air rushing against something and causing a resonance?

there might be a crack or opening where air is coming in. as far as the crack, you might want to check the thread about the cracked dashboards.