Chattering ISX15

[FrenchFry]

I need to get a real answer from a real pro. I have a 2019 Peterbilt 389 with a 565 ISX15 Cummins. Around 9000 miles I started getting a chatter sound when the engine is under load. It's now at 23200 miles and a little worse. I thought it was a valve train issue. A Peterbilt mechanic thinks it's a gear lash in the front of engine. My fuel mileage has dropped from 5.7-5.8 to 4.2-4.4mpg. Also, I believe coolant is leaking into EGR. I need so.e real answers before this brand new truck ends up as junk.