have you recently changed your plugs?
Only use: Denso K16PR-P11 spark plugs others can cause missfire codes. See my FAQ link in my sig below for more general maintainance
http://www.automotiveforums.com/vbul...d.php?t=178212
http://www.automotiveforums.com/vbul...&highlight=ion
If you have a 2000-2004 Isuzu, make sure you only use Denso K16PR-P11 spark plugs. The 2000-2004 engines have the GM/delphi "ion sensing" ignition system where the spark plugs are used as a sensor (when they are not firing) to measure the electrical resistance of the gasses in the combustion chamber. Other plugs may cause the ECU to register a code (misfire code: P1340). Also, it has been noted that some 00 trucks came with champion plugs from the factory, which cause knocking, due to to ion sense coils.
If you have 1998-1999, the suggested plugs are NGK or Denso or Champion double platinum plugs.
A dirty EGR port may cause knocking. Knocking also, may be caused by carbon build-up in the combustion chamber. Adding one 20oz bottle of Chevron Techron Concentrate® every 3k, will help to keep the combustion chamber and fuel injectors clean.
Finally keep that PCV valve clean! Either replace it once a year, or pull it every 6 months or so and spray it out with some carb cleaner.