without knowing which sideof the vehicle the sound comes from and a few other things, i'm willing to tak a stab at it by suggest that you check the following:

A. Heat shield on the catalytic converter (first muffler looking thing you'll see as you follow the exhaust pipe from the engine)
B. Plastic air dam and splash panels under front of vehicle.
C. radiator and fan to be sure they are not loose.
D. with car parked, bounce the car's front end by hand to see if you hear the noise then.