tdi engine nocking noise

hi there anyone help ive just bought a seat leon cupra tdi 150 03 plate its done 180k with full service history but the engine has got a weird knocking noise i know its not the bottom end or anything at top end, doesnt sound like that kind of thing . it will still make the noise untill i get a couple miles down the road then its fine today i took the airbox off and it sounded like someone was kicking a door thorugh the air box i took the pipe off from the inlet side and its full off black crap in there so i was wondering if its anything todo with the egr if anyone can help i be most gratfull