i think its the engine shock. I wedged something in there to keep it from vibrating while someone had it in D with brake and the rattling seemed to disappear. so I'll have to get that replaced

Check your motor mounts, you should be able to clearly see the black rubber underneath the heat shield on the left side, if it is pancaked it needs to be replaced. You can also try rocking the motor back and forth when it is stopped. If it has no give that is also an indicator that the mounts are cooked.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6

Yes, that is it..very common.
It is the top bushing ...the glue that holds the rubber insert to the bushing sleeve dries up , allowing the top eyelet to rattle like hell. By sticking a large screwdriver in between the eyelet and the mount, the rattle ceases.
If you look close , you can actually see the top of the shock moving rapidly sidways. That is the metal to metal rattle..the bushing is actually still good..it is the glue that has dried up. So, there is a very simple solution...You can replace the shock/damper or you can simply sandwich an O ring the size of the eyelet in between the eye and the mount,, Just take the top bolt out, slide the O ring in, like a washer and re-install the bolt.. The ring just takes up the side movement space of the bushing. Several here have used this tip of mine and it last for years....and cost 50 cents.

A very annoying rattle common to all the chassis that used that Tensioner/ Damper system..Later chassis used a different tensoner that did not require the shock/damper.....so it was eliminated on the late 124s..........

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A Dalton

Good advice from Dalton.

I had the same type of problem on my Jeep. It turned out to be the cats...

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!