THUNK noise on 2002 Mitsubishi Lancer

First of all a THUNK noise is the same noise that a fender or clothes washer makes when you lean in and then move back. This noise is coming from the passenger side of my car…opinion varies if it is the front or back. The noise occurs whether or not I am moving straight, turning, or even idling at a light. It is summer in Memphis, so the Air Conditioner is running. This noise does not happen often, but it happens at least once or twice a day. The rear tires have recently been changed. ANY IDEAS? I want to make sure that my car isn’t going to come apart on me in the middle of a quick jaunt on the interstate