So far I have managed to stump the local MB know-it-all and the dealerships on this one...

I have a 92 300D 2.5 with ASD ( locking differential ). There is a fluid management unit of some sort that sits infront of the passenger rear wheel. It makes a click-click sound every few seconds. Over the past 2k miles is has been getting loader. There is a high pressure line that runs from the power steering reservoir over the radiator which pulsates with the clicking. What could be the problem? Any thoughts are appreciated