Hey folks! Amazing group of people here...Just have a little issue that I need to determine its severity ---

I have a knock that occurs when engine is near idle to around 1500RPM. I thought it was rod knock bit its not at the front of the car and seems to go away and come back. When cold, it sounds a bit like a helicopter (chop, chop, chop). All of the descriptive words I use here make it sound alot worse than it is, so imagine that each sound is muffled. I've just never heard anything like this before.

I've heard rod knock, and diesel rod knocking seems to sound twice as loud. Its not driveline-related, it does this in P and N and when under load and not.

I'm leaning toward something vibration related (I need bushings in my shifter and can feel these knocks in the shifter when they happen)

Just to re-iterate, they'll go away and come back in some kind of cycles....I'm thinking its motor/transmission mounts, but the engine doesn't really shake and the motor mounts seem good

EDIT: I found one thread that died without an answer talking about a helicopter sound, and thats the sound its making.

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Java Developer/Linux SysAdmin

Current:
*1984 300D ~200K,1989 MR2 250K, 2012 Ford Fusion 4.5K (fiance's car to replace the uber-unsafe cavalier)
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Past: 1998 Chevy Cavalier ,2005 Saab 9-3 2.0T, 1996 Mazda Protege, 1974 Porsche 914 2.0, 1997 Land Rover Discovery 4.0L/5Speed, 1995 Jeep Wrangler

People have found that making a short video of it and posting it on YouTube for others to watch/listen is immensely helpful with noises.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

I might do that tonight and upload it.

This car really is coming together, and I really like driving it! It's a shame more people local to me dont have W123's...I used to see them all the time but I don't see them much anymore.

__________________
Java Developer/Linux SysAdmin

Current:
*1984 300D ~200K,1989 MR2 250K, 2012 Ford Fusion 4.5K (fiance's car to replace the uber-unsafe cavalier)
----------------------------------------------------------------------------------
Past: 1998 Chevy Cavalier ,2005 Saab 9-3 2.0T, 1996 Mazda Protege, 1974 Porsche 914 2.0, 1997 Land Rover Discovery 4.0L/5Speed, 1995 Jeep Wrangler

Could be injector nailing. Does it sound kind of like hammers being smacked together, or a marble bouncing around. If so try opening each injector hard line at the injector one at a time, and see if the sounds goes away. If it does with one of the lines, then that cylinder is the culprit.

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I think I have the same thing as the OP. If his is like mine, it's more of the hammers smacking each other instead of the marbles. From what I've seen online, I think it's the injector nailing. I think Deisel Giant(or one of the other MB Diesel sites) has a vid showing before/after new injector nozzles and how much it can help. One of the highlights of the video is how it hammers before, and once the new nozzles are put it, it's smooth as can be.

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1984 Mercedes-Benz 300D