Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
Hi,There aren't a whole lot of things in the back of these vehicles that will cause knocking noise so we only have a few areas of concern, some of which you have probably checked already. I am assuming that this is a rather constant noise (either steady or otherwise reproducible and not something that happens "one time" like a loose brake pad, something rolling in the trunk etc.The first thing I would be looking toward is the muffler. It is not uncommon for these to shift or even have a single hanger tear and cause an abnormal knocking sound as you drive. With the engine cool you can grab the exhaust tip and shift it hard from side to side... it should not hit anything. If it does, then there is likely a hanger that has stretched or torn, or the fittings need to be loosened and retightened.The second thing I would be interested in would be the rear sway bar end links. These are the short upright bars that connect the long thin sway bar to the control arms. These are somewhat common to break particularly if the vehicle has any rust.
If nothing can be found there, things will get more difficult to inspect as you will be expecting something suspension related. I would first check around the tires to make sure there is no evidence of anything hitting (like a loose liner) and also make sure the lugs are tight.
From there I would want to get a trustworthy helper to reproduce the noise for you while laying behind the car so you can place a hand in various spots of the suspension to feel for movement/indication of play. If the noise can not be that easily reproduced (like it has to go a considerable distance or over bumps etc preventing you from being behind/under it) then you would need to get the vehicle onto a hoist so that the suspension can be checked out to look for abnormalities. This would primarily consist of using a long pry bar at suspension attaching points to check for excessive play in any mounts/bushings. While under it would be good practice to take a rubber mallet to the bottoms of the tires as this can help reproduce noises like this when the issue is in the strut mounts etc.