Gazdallas

My girlfriend has a C2 vtr with semi auto gearbox. Recently it has developed quite a loud rumble, almost a roar. It only starts when you reach about 30 mph. Went in for mot which it passed and asked garage to check wheel bearings. They are all fine with no play. She took it back to garage to ask them to diagnose the rumble but they can't find anything. They said it was safe to drive. I'm not so sure as it is a loud rumble. Any help would be greatly appreciated. Gaz.