There is an irregular knocking coming that I can feel under driver side foot rest/floor. It is not constant or consistent. Very random in occurrence, it literally sounds/feels like someone knocking right under the bottom where the floor is. It is one or two knocks at random. Doesn't even sound like metal on metal. It's muffled so I only feel/hear it if I sit idling and have to have my feet planted on the floor.

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1990 190E 3.0L

Is your motor jostling about?
Could be a tailpipe.
Get under there and find out what is loose.

Or a squirrel.

If you don't have ramps you can drive one side up on the curb and it will give you some clearance, or find a natural ramp in a parking lot to drive up to give you clearance.
Put the parking brake on!

Motor seems fine as far as mounts and stability. Idle is really smooth. I'll get under it today with a pry bar. Could it be anything internally in the engine? like engine knock? Or I guess I wouldn't feel that on the floor board.

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1990 190E 3.0L

Is it doing this on the move ,or stationary ? If on the move check the main driveshaft centre bearing for play .Or the transmision bracket on the gearbox .

Flex Plate might cause,or convertor.Bolts also get loose,on 617 diesels sometimes front crank pulley will get loose and make you think your throwing a rod.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

You need to reproduce the sound.
Does it happen with the engine off but ignition on? Could it be a loose fuel line moving around because of pressure change? Just a thought.

This reminds me of two things... once I ran over a raccoon on the highway and it hit the car right where my feet were.... gross.

Once I bought a 1983 BMW 633 with a 5-speed (AWESOME CAR TO DRIVE!)
And the steering box mounts were broken so the previous owner used big U bolts to crank it down. It shuddered terribly braking at highway speeds.

This car also had bad camber in the rear, so the tires were worn to the steal belts on the inside. When you pealed out the car threw a shower of sparks... Again... Awesome!

Flex plate would only thump when you put in gear... Right? Not at idle.
But then again the trans is right there by you driver foot right?
Could it be a loose trans cooling line?

It could've been gasoline. I ended up getting a different brand and it seems knock is gone.

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1990 190E 3.0L

A likely cause of such a noise is deterioration of the driveshaft center bearing pillow block, which will allow the driveshaft to flop around.

My center bearing and pillow block were replaced on warranty at about 40K miles because the bearing had spalled and was making a growling noise.

At about 80K miles my 190E 2.6 developed a squeaking noise from underneath. It was most noticeable at about 15 MPH. With the car on jackstands and the catalyst heat shield removed I was able to reach up, grab the driveshaft, and wiggle it around, and then saw that most of the pillow block rubber was gone.

Detonation is most likely to occur under low rev/high load conditions and will be more prominent at high coolant and/or ambient temperatures and won't sound like its coming from under the car.

Duke

Interesting, once I changed the fuel, the knock was gone and hasn't returned. However, last night I did replace my support bracket because I know my driveshaft has been off balance and the rubber was deteriorated. The knocking occurred when I was stationary and there was no movement so I had ruled out the bearing even though I knew I needed to replace it. Finally got around to it last night, but I know it will need replacing again until I get the shaft balanced.

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1990 190E 3.0L

So this irregular knocking was actually the bottom portion of the harmonic balancer skimming the top of my sway bar (400E), problem was resolved by adding 2 bump spring perch pad and lifting the front end a bit.

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1990 190E 3.0L

Sounds like it’s time for new engine mounts.

Sixto
98 E320s sedan and wagon

Yup, amazed me how compressed my mounts were when I changed them out. At least 1" lower than the new ones.

... and, removing the W124 V8 sway bar, and installing the proper W201 Sportline / 16v sway bar.