On a recent 700 mile family trip, our '07 Navi L developed a strange sound from the dash. Initially it sounded like a blown-out speaker crackling, but it only does it at high speeds and it will still do it when the radio is off.

I believe I've got wind coming through the firewall or an air intake somewhere, and it's hitting something thin that just flutters in the wind... giving me this terrible crackling sound.

Any guesses where to start?

We're driving 350 miles tomorrow and another 350 in about a week, so I'd like to get it quiet!

Thanks,
Greg