i need to get some input so i can steer my dad in the right direction, hes got a cummins 8.3 diesel in his straight truck, and its developed a slight knock. it only does it when he revs it up or is holding at any sort of higher revs, and if its in neutral you only *barely* hear it while revving but when its under a load in gear you can hear it nice and loud.

i was thinking a main bearing maybe, cause under a load there is deflection and thats the problem. other options are a possible rod bearing, or something in the valvetrain. he did say it didnt sound as fast as the engine is spinning, so hes wondering if it could be a valvetrain thing or could even a bad injector cause it to knock? he also said it does feel like it has a miss though, which goes against my thinking of a valvetrain issue.....

any diesel mechanics around here? help would be appreciated as hes on a 1200 mile run right now and when he gets there he's gonna try and tackle whatever the problem is.