very strange noise

Well this one has me beat , not being a deisel mechanic and my first landy, when the engine is running this noise cannot be heard but when the engine is under load it is quite loud it sounds like a drrrrrrrrrrr noise which sounds like its coming from the back of the engine ..... The engine is a two and a quarter standard in my 1983 series 3 , it sounds like a broken follower , but i know they are all ok and the tappets are all set correctly .The valves are all ok , checked them . replaced the injectors , pushrods are not bent, but still get this loud drrrrrrrrrrrrrrrrrrrr . Seems to run ok 65 onthe flat runs and pulls ok off road to but the noise buggs me , any ideas , and dont suggest ear plugs thanx mike