This is a common issue on OM60X engines. If you raise the hood and watch the tensioner shock and spring while you increase the engine speed you will be able to see the shock vibrate and emit the noise you describe. On my car the noise appears at about 800rpm and disappears by 1000-1100 RPM - a new shock solved the problem everytime.

I seem to have to change my tensioner shock every 70-90K miles - I've never replaced the spring.

Tim