+1 on RadioTek's answer. The last time I had that sound it was a tread seperation in a Firestone tire...

Note: If you have used a Instant Flat Fix or other tire sealant / balancer product they can cause a similar sound. It would be most noticeable when the tire is cold and usually goes away as the tire warms up...

Wayne

2008 Toyota Camry Hybrid, Magnetic Grey, XM Radio, Body Side Moldings, Splash Guards, Formula 1 Pinnacle 35 Ceramic Window Tint, PadsPlus Neoprene Rubber Insert Set, Putco Chrome Door Handles and Trunk Trim, Michelin Primacy P215/60R16 95H Tires, SE Rear Spoiler