Are you sure it's not a whistling sound?
http://www.986faq.com/10-0/problems.asp#012
Whistle While Driving
Diagnostic: Whistle sound proportional to speed while driving.
Cause: Wind blowing over gap.
Details: The sound can be caused in several places, but the most frequent is the front edge of the hood. The solution is to have your front hood height adjusted. If the whistle still is there, you can apply a gasket where the front gap appears.
But Mark_T has the best diagnosis, you propably have clowns.