Knocks from under the car
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Joined: Oct 2004
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From: Kansas City. U.S.A.
2000 E320 Sedan
Knocks from under the car
I have e320 yr 2000, with 56,000 miles. Last month, both CATS have been replaced thinking this issue is related to the CATS. Well it appears it is not. The issue is after starting the car in the morning, and after the first idle is done "1-2 minutes after startup", the knocking sound begins and it lasts for about 5 minutes and then goes away for the rest of the day. It appears to be coming from under the car somewhere between the 2 CATS, what could this be? Pleeeease help. 
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From: Kansas City
97 E420 90 300E
I had a problem when one the mounts for the exhaust broke off and needed to be welded back on. It would rattle at Idle, but if yours is more of a knock??? I dont know what part of the exhaust would really make a knocking sound.
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From: Kansas City. U.S.A.
2000 E320 Sedan
Originally Posted by kevins e55
resignators? and are you sure its coming from under the car and not the engine bay?
What are all of the scenarios? Where is the resignator? and what would be the cause if the sound coming from the engine bay?
Thank you
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From: Kansas City. U.S.A.
2000 E320 Sedan
But I always used 91 octane Premium unleaded from QTRIP gas stations. if it is gas related, the car will not run very well, but it does run great. Only the morning knocks that bothers me.
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From: Riverside, CA
98 E320
Originally Posted by cragigotgame
I had a problem when one the mounts for the exhaust broke off and needed to be welded back on. It would rattle at Idle, but if yours is more of a knock??? I dont know what part of the exhaust would really make a knocking sound.
In my case, the sound was rattling and it only happened while in gear, at idle, like at a stoplight or drive thru window. Not at highway speed, and not at idle while in park.
It was a broken weld on an exhaust bracket used to separate the pipes. the reason it rattled while in gear makes sense; the pipes share a bracket on the tranny and while in gear, there is engine/tranny movement. While in park, there is little or no movement, thus no load, and only little movement.
The break was barely noticeable and I only found it after wedging the pipes apart. If you look at it with no stress or pressure on the pipes ~ the weld looks good.
Figure out when you are getting your noise. In gear but at idle, or in park at idle. My sound disappeared at highway speeds and tracing a sound in the exhaust system can be misleading since noise travels up and down the pipes. Literally took me several hours to find the broken weld.
My neighbor put a quick weld on (it's carbon steel and can be welded) and has been fine ever since.
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From: San Jose, CA
2000 E430 Sport
Exhaust
I had a problem with my cats also. I replaced them both and noise went away. I started to hear a rattling sound coming from under neath the car a month later. What I found is that there is a metal ring causing the sound, it goes in between the flanges. It is the flange that connects the Cat to the resonators. Check it out. That was my problem and had it fixed
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From: Kansas City
97 E420 90 300E
Yea it sound like its probably somthing vibrating underneath. I would take to an independent german car shop. There are a few around. Maybe even star motors. But I would avoid arisocrat.
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John Deere
Originally Posted by cragigotgame
I had a problem when one the mounts for the exhaust broke off and needed to be welded back on. It would rattle at Idle, but if yours is more of a knock??? I dont know what part of the exhaust would really make a knocking sound.
I have this same thing now. Need to get under there to find out exactly where. Mine is a jingle/rattle mix at idle.
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From: Kansas City. U.S.A.
2000 E320 Sedan
Originally Posted by mike430sport
I had a problem with my cats also. I replaced them both and noise went away. I started to hear a rattling sound coming from under neath the car a month later. What I found is that there is a metal ring causing the sound, it goes in between the flanges. It is the flange that connects the Cat to the resonators. Check it out. That was my problem and had it fixed
wHAT IS IT I NEED to look for when I look at it. A warn out ring or rubber arounf th ring? what is it exactly. Do I need to replace thr ring regardless ?
Do you have a picture of what to look at at?
Thank you
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get a rubber mallet get under there and hit the exhaust it wont hurt anythng, listen for the sound, if no sound drive the car and then do the same but be careful cuz it will be hot, if still no sound then its not the exhaust, sometimes the catalyst that came apart in your OLD cats could have been blown into the muffler or resonator, also check trans mount
