I see you bought your Mazda2 used with 29,000 miles on it. I would first question the history of the vehicle: Has it been used and abused? Has it been off-roading? Has it been in a wreck? To me, the clunk sounds like a steering knuckle or CV joint is worn. For it to happen so quickly after the dealership fixed it, it makes me suspect an accident or abuse that has caused some sort of misalignment between the front wheels and the road. I would consider taking it to a mechanic (non-dealership) that will look at it for free and let you know what's going on. When they tell you what's wrong, that's when you can take it to the dealership and see if it's covered under the remainder of the warranty.