Well, I took my car to get checked and everything under my car is tight, but the sound is still there. I noticed that the passenger side is a little bit lower than the driver side (about a half inch), so I'm thinking my spring should be replaced (going on 280,000 miles). Am I correct in thinking that a worn spring can cause a thud in the back? If I go straight and hit a bump normally, there is no sound. However, if I go fast over a bump or hit a bump while turning left, you hear it.