Sounds like a cracked piston and subsequent rod knock. I have a running theory based on my experience in the field is that it's LSPI caused by well, certain oils. BMW got ahead of the problem with their newest LL17FE+ oil spec that resist LSPI, other oil brands got certed for API SN PLUS is the API's new spec for LSPI fighting oils. I noticed certain oils retconned their API SN Plus without changing the formula or debuting new product so I'm a bit skeptical. How could a decades years old formula with every shiny manufacturer approval under the sun that cooks off a quart casually in naturally aspirated cars have foreseen LSPI events in the future? It's not a problem exclusive to MB. The Kia/Hyundai 1.6 have an awful time with premature turbo death and LSPI events destroying engines. I'm wrong about this until I'm not, I just want to get ahead of this in case my delusions were right all along.

Drain the oil and pull the filter, check for shrapnel. Compression test. Don't drive or start the car anymore so that saving the engine block is on the table if you want to replace the cracked piston if thats what it ends up being.. If you're under warranty or youre the first owner start making phone calls.

I could also completely be wrong about this.

Thanks ChrisHimself for the reply I really appreciate it. I hadn't considered a cracked piston so I will do a compression test. Yesterday I narrowed it down to cylinder number 3 but nothing conclusive just yet.

That is interesting because it is usually cylinder 1 that goes if or when this occurs.

Your post reminded me of this :https://mbworld.org/forums/glc-class...ml#post8716907

yeah i mean hopefully im very wrong and its like open vacuum lines or something goofy, but i mean anytime theres like knocking on decel it's not great.

yea i know, i've seen it in the shop. people actually break down and cry it's really sad.
The industry knows about LSPI and everybody is like ok theres an API SP to address timing chain wear, LSPI, but then now theres API SN PLUS spec that REALLY fixes this time and supercedes it. Suddenly, and all the existing oils are like YUP ME TOO and it's like... then how tf do we know about LSPI, timing chain wear, spark knock, turbocharger wear then if all the oils on the market are hunky dory and full synthetic and super great and completely unchanged from like 20 years ago?

It's like oh we're the approved factory fill for Porsche so we're the best. Ok then why does 15% of my oil fill end up in the atmosphere?

Sure is confusing as a whole.