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Re: rattle under car going over bumps
Take a close look at your springs. My 99 Audi A4 had a creaking rattle that happened when I went over a speed bump or dip in the road, and it turned out the bottom of each rear spring was broken. I replaced them and the noise went away.
Another possibility would be the sway bar end links. Since the sway bar is worked pretty hard, the bushings in the end links will wear out eventually. This can cause some clanking. Look behind the wheel hubs - you should see a short bar or bracket attached to the hub at one end, and to a bar at the other end. The bar will go forward or back a short way, and then across the bottom of the car to the other wheel hub. That's your sway bar. The bushings in the end links (the short attachment bars that anchor it to the wheel hub) or in the body brackets (some cars bold the center of the sway bar to the body with a bushing in a bracket) could be deteriorated and allowing the bar to bang around on the body.
Finally, take a look at the suspension itself. Do any of the bushings where various parts meet appear to be cracked, leaking fluid, or gone entirely? There should be no gaps between any suspension pieces at the points where they meet. If there are, you're missing some bushings and may well have found the source of your noise.
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Even though it sounds like it is under the armrest I would suspect the noise is a exhaust system heat shield that has a broken tack weld that is the source of the noise, raise the car off the ground and with a rubber mallet tap the exhaust heat shields, if you find a loose one buy a large band clamp (avail at hardware stores) and put it around the heat shield and tighten it up, the noise will then go away.
The metal shielding around your catalytic converter could be rattling or the rubber donut that holds the rear tailpipe could be bad. When your car is cold shake your tailpipe and see if you can reproduce the noise. If you can your tailpipe donut is likely bad. Three dollars at any auto parts store. If it does rattle when you shake your tailpipe then the heat shield for your convert could be loose. You need to tighten it up another piece of metal between it and the converter to keep it from rattling. If you hear the rattle when you go over bumps too then your exhuast is probably, loose. Crawl under there and stake shaking things, I am sure you'll find the problem
The heat shields that are positioned between the exhaust pipe and floorboard or between the exhaust pipe and sensitive electrical components are made of very cheap, thin, tin-like metal. When they rust on one side, one of the tabs comes loose and the whole shield will vibrate especially when accelerating. Since you are getting the noise when you hit a bump, you might check the area where you stow the spare tire (loose tools may be vibrating against the metal). Also the plastic protective pieces behind front bumper sometimes tear and will "flap' against the car and sound like a metallic vibration when you hit a bump especially if you were stuck in snow recently. Look for loose plastic pieces.
Check your exhaust and rear suspension for something loose or broken. Problem could be the exhaust, a hanger or cracked pipe. In the suspension it could be shocks, worn bushings, track arms etc. When a problem like this happens you have to get under the car and grab and try to move things checking for loosness. In general try to see what is loose enough that a bump causes it to rattle or bang. Other than inspecting the rear area there is no short cut that can be taken to solve the problem. You don't want to just start replacing parts hoping to fix the problem, that can be expxnsive and unnecessary. Good Luck!!!
while driving at higher speeds you may not notice a sound as much as when you are drivng in reverse as you usually drive slower in reverse. This leads me to think that the rattling can be caused by a loose protective plate under the car. When a load is put on the engine, it tends to vibrate more this vibration can cause a lose plate to rattle more. This is the cheap answer. That is if you are definitely sure the sound is coming from under the vehicle. Otherwise, you may have a crank shaft problem or a seal in the bottom half of the motor could be bad. You may want to crawl under the car and try to shake some of those plates underneath. If none of them are loose take the car to BMW it shouldn't cast anything for them to look at it unless they have to hook it up to the computer.
Please remember that if you intend to climb crawl under your car, you must make sure that your car is properly supported to prevent serious injury. Also, if your driving with a loose plate, it could easily fly off while driving or if you hit a bump. This could cause serious damage to your vehicle. Also, please check you oil levels and make sure you do not have an oil pressure light coming on when you hear this noise.
Yep, there's a good chance some part of the exhaust system has come loose and is rattling at certain RPM's. Start fault-tracing by checking if any part of the exhaust seems loose to come in contact with anything, or if there's any loose clamps or similar parts hanging onto it. Another quite common reason for rattle is that some part of the exhaust manifold, such as a bolt or hose/pipe has loosened (often due to rust and/or corrosion).
Another possibility is that it's somehow related to the transmission (since you mention putting it into gear). If so that would be another subject.
Check the exhaust system. When the car is cold, jack it up, place on safety stands, block wheels, crawl under and grab on to the exhaust, starting at the back of the car, give it a good shake (this is why you block the wheels, put it on safety stands, u don't want it falling on you). Work your way up to the engine. Many times it is a heat shield that has rusted and is loose that is making the rattling noise. There is no harm to the car, just annoying. You can usually use wire or a couple of screws to secure the rattling part .