Mercedes: C200 Saloon Tapping noise, fuel delivery system, fuel pressure regulator

Question
Hi Rob
Any idea what causes a tapping, knocking noise at idle speed.  Sounds louder under back seat and only audible inside the car or underneath with a stethascope on the fuel line.  We have replaced the fuel pressure regulator but the noise continues and is louder when the engine is cold.  The frequency of tapping decreases if in gear, but slighter faster in Park or Neutral.  Recently replaced weepi9ng rubber seal on fuel tank bung.

Answer
IF this vehicle has an internal fuel pump in the tank, there may be a check valve rattling. If the fuel pump (s) are external then you should be able to place your listening device directly on the pumps to determine if that it the problem.
Have someone increase the engine speed in park while you listen with th e instrument, to see if the knocking increases in frequency with revolutions. This increase with rpm would indicate something in your fuel delivery system is causing the noise. The system may have a check valve to keep it from bleeding back to the tank when shut down. There may also be a possibility that the automatic transmission pump is clicking, not as likely though since it seem to be fuel line connected.
Good Luck! If you are listening to a metal line, there may also be a possibility your are hearing the noise by mechanical transfer from the source through the steel walls of the tube.

Let me know I you find a fix...
Rob