resonating

Mildly facetious question, I am assuming its primary purpose is to muffle noise from the engine, but I'm also curious whether there's any other purpose. I've discovered that the engine cover is responsible for the majority of unpleasant noise from my engine - resonating as the engine...

hi - my 02 C180 automatic makes a resonating sound when on the motorway when get to the 60-75mph range. it comes from the back, i'm guessing from the driveshaft, it doesn't make this noise when driving around town or going through gears getting up to 60mph and doesn't do it when go through...