I recently bought a really clean 99 midnight blue C280. While it is clean it was poorly maintained and is just showing signs of its latest problem. It makes a sound like blowing across a jug or a rubbing sound that resonates in the whole car. The sound seems to appear when going up any grade at highway speed. It goes away on the downhill side. I tried to alter it with a steering adjustment but finally discovered that a slight throttle adjustment can quiet it. The engine really purrs like a kitten at idle and moves like the wind when asked without touble. What could cause the noise, a pulley on the belt? I replaced the belt and tensioner a month ago. The sound is about a week old.

Thanks, Pete