what i woud do is pul the egr valve and see if its clogged or stuck open, clean it , i usually dont tell people that but if you dont have the money try to clean it , make sure its not hung open and try it , the reason for the noise is like in a gas engine , its like a spark knock but a hel of alot louder in a diesel , im not sure if thats your problem or not , but try what i said and see , let us know