One possibility is that the plumber used Qualpex (plastic) pipes but didn't use enough hooks to fix it in place. The pipe is so light and flexible that when the water passes through it rattles off the surrounding surfaces like floor boards etc.
A little bit of humour to brighten up a Friday afternoon!
Anyway to answer your question: the noise is referred to as "water hammer" and is most likely a result of inadequate clipping of the pipework, as suggested. Try and identify the location of the offending length of pipework and clip or lag this. This may require lifting a floorboard or two.....
Actually a quick search on wikipedia gives a far more comprehensive explanation behind the phenomenon- basically related to the kinetic energy of a moving fluid. There may be a simple fix: water hammer arrestor. Could be worth talking to a plumber or plumber's merchant before you consider lifting any boards....