2012 Isx 15 rattle noise top driver side

[Broncof15079]

I have a 2012 T700 with cummins Isx15 450hp 488,000miles, 1 month ago it started to rattle. No engine codes When the truck starts to pull a grade it will start a rattle really loud it sounds like its on top of engine, and only around 1400 to 1600rpm , if I let off on pedal it goes away, when I get back on the pedal rattle comes back ( sound is like bearings or valves coming apart). So far mechanic done dpf and doc filter clean and burn, overhead adjustment and has not been able to solve problem..
Please help