stevens cummins

ive tryed recreating the noise and finding it by having my gf hold the rpms at where it makes the noise and me under the hood and by ear or with a mechanics stetha scope i cant hear it ive searched the whole truch over for loose items i even adjusted the valves and looked over all the rocker arms valve springs and bolts under the valve cover i even ran 3 tanks of fuel with injector cleaner tru it and no help

the noise is at most all rpm above 1000 im thinking fuel but also am concerned that it could be something to do with my exhause due to the fact that i installed a 6in single stack back in the summer but i havent been able to find any info on that causing any probs however im still running the stock down pipe then it turnes to 5in under the cab the to 6in tru the bed. i also know the vp44 oump was replaced at about 120k before i bought the truck and it now has 160k it also had a low fuel psi light installed then which has saved my vp44 more than once in a year cause ive had 4 lift pumps fail it sucks settin on the side of the highway changin them

ok i have a 2000 cummins it runns good and idles perfect:thumbup2: but when im crusing and just in the throttle just enough to maintain speed it raddles up a storm almost sounds like a little jackhammer:neutral: it goes away when i release the trottle it also goes away if i hammer down:woot: but then i get what sounds like a random pop comming through the stack:stuned: i have adjusted the valves and checked all that i can think of:banghead: does anyone have any suggestions as to what it could be