A car making a clunking noise could have one or more of a number of problems, including problems with the transmission, exhaust system or wheel bearings. The first step to determining what is causing the sound is to isolate where the sound is coming from. It is also helpful to figure out when the sound occurs.
If the car makes a clunking sound when it is put into gear or when the gears are changed, this indicates a potential problem with the transmission. If the sound occurs when the wheel is turned, the problem is most likely with the wheel bearings. If the clunking sound increases with acceleration, a hole in the exhaust system is one possible cause. When the car makes a clunking sound followed by vibrations during braking at high speeds, have a mechanic conduct a thorough check of the braking system.
If the clunking sound is constant, this makes the cause harder to diagnose. If the sound is accompanied by a change in the alignment of the car, the axle might be bent. In addition, this is a common result of the frame of the car being damaged and rubbing up against other parts near the wheel well. Although it is a fairly uncommon cause, engine knocking can occur from putting low-grade gas in a car.