Knocking Sound when running on 300d
 

Hi all,
I hear a bit of a knocking sound when my 1884 300d is running. It's not terribly loud and I can hear it better when under the car listening to the engine. There's not a regular rythm to it so I don't think it's a broken part. Any ideas. How loud would the Vacuum Pump be if it was doing it's breakdown routine (the common knocking it's lever makes before it blows the engine to shreads:) ?
I don't think it's that but it could be just wanted to see if anyone had ideas...