2001 Ford Focus Exaust Roar and Vibration?

When I start my 2001 Focus, car vibrates and as I drive, it sounds like a hole in the muffler or an exaust gasket, etc. After driving awile, it goes away (15 mi). The car has been checked by two different mechanics and they say there is no exaust leaks. Who has had this problem and had it repaired successfully. Auto has 75000 miles.