Several years ago I read an article on diesel fuel. It concerned a new refineing process called hydrating, the reason for the new process was to lower the sulfur content in diesel fuel thus lowering emmissions of particulates from the exhuast. The process of hydrating the fuel also removed the lubricating qualities in the fuel. I only know of one company that puts an additive back in the fuel at the refinery to correct the lubrication problem and that company is HESS.
The additive Redline also adds back the lubrication quality to diesel fuel as well as increasing cetane, cleaning injector nozzels, disapating water, and helping block the growth of algae in the fuel tank.
The noises you hear from the injectors is most likely due to low opening pressures and nozzles that are not atomizing the fuel coorectly. Maybe you should let your mechanic road test your car and see if a problem exists, and if so, remove the injectors and bench test them for pressures and spray patterns. Replace the weak or bad ones and your noise should be gone.
Hope this helps,
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop