Toyota Camry 2009: Auto transmission shuddering noise

When we tested the 2009 Toyota Camry Altise before we bought it everything seemed okay, but soon afterwards we heard a shuddering noise in the auto transmission when selecting reverse or drive. We've had the car back to the dealer twice for this problem, but they have told us the transmission is okay and that the problem is most likely the torque converter. What is my legal avenue with this dealer to get this problem fixed?