Lets face it. Diesel engines make noise. With all the engine knocking and vibration it is sometimes very difficult to isolate the source of an annoying noise. I have already covered using Diesel Purge to diagnose internal vs external noise. What if you can't really determine where it is coming from. Those are the most difficult to diagnose.
Problem & Solution
Common among these chassis:
For these types of noises sometimes experience is your best teacher. From time to time I will post information on sources of noise I have found on diesel engines. Here is one you may not have experienced or heard of before. The engine driven fan clutch can be a source of a number of different loud and obnoxious noises. If the engine is producing a roaring noise at high speed it may mean your fan clutch is seized (not allowing the fan to slip). I have also seen fan clutches start to break up inside and actually cause knocking noises that sound like a loose timing chain tensioner or rattling valve train. If you hear similar noises in your engine be sure and check out the condition of your fan clutch.
Look for signs of rust or staining on the back side of the clutch and fan blades (see picture). That could be an indication your clutch has been leaking fluid or has run dry.
For troubleshooting other diesel engine noises see Kent's manual listed under related products.