If it is only a slight thump and it is only there when the tire is cold, it could be that the tire has taken a set from sitting overnight. If that is what it is, it goes away after driving a mile or two. Tires with nylon cords in the sidewalls are the most prone to this. The Goodyear Eagles used on early Mark VIII did this and Ford's recommended cure was to install Michelin tires.
I am more prone to think that you do actually have a tire problem though. The advice to check the tire condition, especially for bulges and seperation, is excellent advice. Michelin has not been ammune to such problems.
