Engine Noise SLK350

[Dave200]

I have a continually repeating whirr-click from top of engine. Slight affect on RPM as though putting on a load then releases after click. Repeats every few seconds. Has done this for a long time. I've ruled out auxilary air pump by unplugging. Any ideas? Maybe they all do this? Thanks Dave

 

[LostKiwi]

Try turning off the air conditioning and see if it does it.

 

[Blobcat]

Is it the idler pulley on the auxiliary belt? That sits at the top of the engine, slightly to the drivers side