We have a 2016 Peterbilt 579 ISX15. It had a cracked head, replace that and the gasket. After the shop put it back together it's now making a loud knocking sound when you simulate putting it under a load that we can't identify (low RPMs lots of fuel). They've check the crank and scoped the cylinders, we've replaced injectors. Don't know what else to do. While trouble shooting the truck wouldn't start after a couple days because the DEF had a broken switch of some kind. They've fixed that and it's done a regen and starts now. The shop has had it for 2 months. Everyone is out of ideas and we really need our truck back. Has anyone run into this issue or what it could be? It's a loud, out of place sound we're all unfamiliar with so I'm not comfortable with it leaving the shop, but just don't know what else to check. Open to any ideas!