strange noise from the engine

Hi guys,my freelander td4 2003(BMW)has been running smoothly after reconditioning the turbo and changed the block as the original one had cracked.Recently I began to hear a strange noise from the engine,it's like a nocking sound but it comes and goes.The car doesn't smoke but when that noise comes,it affects the power of the engine,when gone it runs smoothly again.even in neutral,if revved its sometimes heard.Anyone with a clue please help.