You probably have worn bushings in the shock absorber on the serpentine belt tensioner. When the engine is cold carefully press down on the top of the shock and I will bet that the noise will go away. You may have to remove the fan and waterpump pully to remove the lower bolt. I just changed them on my '93 300SD and '95 S350. Replacement bushings are not available. Mercedes charges $76.00 list and $61.00 mechanics net for this item.

The fuel supply line is the one with the small "L" shaped fuel filter. The return line is the one that has an increased diameter in the center. Both attach to metal tubing on the inner fender splash pan.