engine knocking noise

hi all ive had a shed loads of problems recently with my 2002 freelander td4 auto,ive just had all injectors replaced,new drive shaft,new alternetor,new prop shaft etc,ive still got a knocking noise and black smoke coming from exhaust,if its left for a few days it will drive good for a day or about 150miles then the next day its back to normal with the noise,shuddering and black smoke im at my wits end and would really appreciate any advice many thanks spencer,