weird noise

as i was driving today, i noticed a weird noise coming from the air vent, in the center, on top of where the stereo system is. it sounds like something is loose and dangling there, but i dont know, didint check yet. And i noticed it happens only when i go at speeds above 30mph.
i got 1998 xlt wiht 4.0 sohc, any suggestions would be helpfull
thanks

as funny as it sounds, the problem got fixed on its own. i was driving my car and just reved up the rpm to like 5-6k and the problem went away, i dont know what the hell was it but as long as its not there i am happy