Continuous Popping of Charcoal Cannister???
That canister shut off valve can be a real pain to get at sometimes, but since you describe it as being so loud, I definately think they should change it out and see what happens.
Keep in mind that Mercedes isn't always forthcoming with information. Sometimes we can't find work instructions, diagnostic info, wiring diagrams, etc... And we are the DEALER! Sometimes we fly blind and figure it out as we go. That takes longer and we lose money that way. Point being that I for one do not even know the strategy Mercedes uses to activate that valve. I know its there, I know what it does, but as for exactly when and why they do it that way, I am clueless. Sometimes they thump, if its loud, it should be replaced and then see if the new one is any better.
Sometimes working on mercedes is like



