SICAMG

Hey guys I have a very strange thumping sound over even the slightest of road cracks or bumps on my 2015 C300 4 matic. Have had the car for 5 years and it has been doing it for the last 35,000 miles of ownership.Been to the dealer multiple times,a friends Mercedes shop, my shop many times and nothing can be found. Replaced the rear shock at the dealer but no change and all components look perfect, all bolts removed and retorqued but no change and is actually worse now than it ever was so at a total loss with this.
If you were to drive over the slightest road crack..say 2-3 inch wide and 1/2 inch deep you will always get what sounds like some one in the trunk with a rubber mallet softly tapping the trunk floor making a thump sound every time and is only on the rear passenger side..... always. Speed makes no difference but slower is notably louder. The K member / mounts looks good as well. Bouncing suspension does nothing to duplicate the noise and everything in trunk tight and secure.
Anyone ever seen something like this?