merc E 300 td diesel knock?

1998 R reg Mercedes E300 TD, 47000 genuine miles, FMSH

the engine in my E300 td has started to knock and ive been told that it sounds like a faulty injector which over fuelling.
it knocks at idle and at 1250 rpm but disappears when revved beyond 1250 rpm and runs sweet as a nut. let the revs settle back down and its starts to knock again when they reach 1250 rpm and back at idle.
starts first time but it feels slightly less powerful than before when driven hard.
can this be a faulty injector only? or can a faulty pump cause it to over fuel aswell.

Peter
April 2014