MalteseFalcon

My focus 1.8 diesel sometimes does a rattling/knocking just like a petrol engine with no fuel and it does it only on 4th gear when i'm driving about 2000rpm. I would give it the 3rd gear speed a bit up and than just put it in 4th and no problem. I would repeat the same situation over and over again and it would not do it. this happens just once in while but makes me mad all the time!!!!

Any one occurred this problem before? any help is appreciated thanks