Good morning fellow users,
I recently acquired a 2010 C300 Sport RWD with 80k miles and I'm experiencing a very frustrating problem, that nobody can diagnose?

When the engine sits for more than an hour, the bottom of the engine makes a knocking sound for 2-4 seconds on startup. It sounds like the engine is starved for oil, sounds like a dry start.
The idle seems to be a little lumpy when the knocking sound stops.

The engine is smooth and quiet at idle and while driving. The car has Mobil 1 5w40 euro oil and a Mann oil filter, I changed the spark plugs , but the ignition coils are old and the throttle body should be cleaned, but I don't think that would cause knocking.The car has no trouble codes.
After it's been started , you can start the engine 30 times in a row and the sound will not reoccur.
The sound is worst and occurs more often the hotter the outside temperature.

I've noticed the oil filter and housing seem to have less oil inside when the engine is cold, and i remove the filter, compared to others Benzes and BMWs I've had, but I'm not sure if that is related.

I've searched the internet endlessly for a solution, I've heard everything from wrong spec crankshaft bearings, cam phasers, air injection pump, etc
If it was bottom end knock why would it stop after a few seconds , once oil pressure builds?

If it was the cam phasers, why would the knock come from the bottom of the engine, not the top?
There are no rattling sounds, my timing chain appears to be tight and clean.

Posters on the w164 forum and the slkworld forum
Had this same issue and it was solved by replacing the oil pump pickup tube o-ring.
On many Russian forums discussing the M272 engine, they also point to the pil pump pick-up tube o-ring as being the culprit.

https://mbworld.org/forums/m-class-w164/804237-w164-ml350-m272-oil-pump-pick-up-tube-o-ring-replacement.html

https://www.slkworld.com/threads/272-engine-knock-on-startup-main-bearing-knock-fixed.586425/

If the oil pressure was checked via a gauge on the front of the engine at startup, would that tell me if the o-ring is defective?

I am located in the deep south and I've called many shops in Houston TX and Austin TX, nobody has yet to even listen to me explain the problem, or they dismiss my concerns, or refuse to look at the car.

I want the car fixed, I will drive it to another state if necessary.
Should I go to the dealership diagnosis and go from there?
Should I call more Indy shops ?
Should I request a quote for oil pickup tube replacement, assuming that's the issue?

If anyone can help me, it would be greatly appreciated, I don't know what to do at this point.
If anyone can suggest a shop in Texas that would at least listen to me or diagnose the car, I would be grateful. I apologize for such a long post, but I wanted to include all the data.

Thanks again

If you're dry starting, there could be evidence of cylinder wall scoring. Maybe find a mechanic who can borescope the cylinders first, before diving into a pickup tube repair?

Place your phone or iPad down there and start recording. Have someone start the engine and get the sound recorded. Rod knock has a very low frequency, syncopated “thump” to it. If the sound is easier to capture above engine it is likely valvetrain.

If this is the warm (not hot) start noise, it's at least innocuous. I've had that noise since new and currently have ~258k on the clock. I certainly wouldn't tear into an otherwise healthy engine, if that's what you're asking. If it matters, my car has only been on a diet of Mobil 1 0w-40 its entire life, although I did switch the fleet to M1 5w-30 ESP recently.