My wife's car (2009 SLK350) is making a weird noise upon acceleration. It almost sounds like she put a lower octane or bad gas in her car. However, the gas is not the issue. When you step on the gas there is this hesitation/flutter noise that occurs. There is a very slight knock in the engine as well upon acceleration. I'm thinking this could be an EGR valve. However, she is not receiving any dash error lights at all. Has anyone experienced this in their SLK350?