If you have an exhaust manifold gasket leak generally you can hear is quite distinctly in the area around the manifold. In my experience a manifold leak does not change the tone of the exhaust system. I am new to Mercedes but have been working with cars for about 50 years. I have seen engines that run too lean make a louder than normal exhaust report. Check your plugs to see if one or all of your cyls. are running too lean. While you are at it check the timing. As you advance the timing the cyls. hit harder and the exhaust report changes.