Jokes aside,

When accelerating from a dead stop (mode does not matter), it literally feels like someone rear ended me the Thud is so loud. It only happens on more than light throttle from a stop. Here is the kicker...

My local MB dealer has had it for over 3 weeks trying to diagnose it. They experience it every time as well. The car has 122,000 on it (2018 C43). The engine and trans mounts are new, the Transmission has been serviced twice in its life now (60K and 2,000 miles ago). Its been happening since around 118,000. My advisor recommended reaching out "to the forums" for advice which I personally find unprofessional but hey... If it helps, it helps.

Before I give up and take it to another dealer, has anyone actually experienced this exact symptom, and if so, what was the fix?

Thanks all!

What kind of sound does it make? Metal, fabric, plastic? Is it mechanical, body, or trim?

Mechanical. It sounds like you’re neutral dropping an old Buick…. Best way to describe it

I'm no expert, but wasn't there a software update that was issued partly to deal with this occurrence (even though it wasn't as frequently occurring as you describe).