I have an 04 Dodge Cummins with a faint engine noise?

[Dustin]

I bought my 04 Dodge CR Cummins in January of 2011. It had 32K miles on it then and has 45K on it now! It has made a faint tick/knock type sound sence I've owned it! This sound, however, ONLY happens when the truck is IN GEAR (Drive, it's an auto) the noise isnt always constant in gear either, it seems temperamental at times! I had a guy tell me to try fuel additive in it so I used Standadyne and it helped in a minor way! I've also used a CRC fuel additive solution that didn't seem to help! I have taken this to a mechanic buddy that re-adjusted the valves with no help of takin the noise away! I also had the dealership take a listen and they found nothing! Just told me to use an additive because an injector(s) may be getting more fuel than the others! My buddies computer he had from workin at ATS said nothing and the dealer did isolation test and other tests and told me to go! I've read about injector problems a lot with my year Common rail Cummins so think it may be that! I would live to have this taken care of before the entire motor is at stake! What are your thoughts??!!