Diesel Bombers - 6.7 Knock

Hi, new to the forum, 1st post.

2008 2500 6.7 Cummins 150k. EGR delete. 5in turbo back exhaust. Cold air intake. Smarty tuner, was on setting 3 (60 hp) when I got it. I've put it back to stock.

Truck developed knocked about a week after I purchased it.

Took it to a local diesel shop and they turned off the injectors one by one. Knock went away when they turned off 6. They told me it was a bad injector.

They put a rebuilt injector in #6. Knock was still there. $500

Then they told me that it's possible I got a bad injector. They said sometimes they're bad right out of the box. So they switched 1 and 6 and did a valve adjustment. The knock stayed in #6. $300

They scoped 6 and did not see any damage. 6 has good compression. Injector balance test showed 6 at 100%.

No blow by. Sent an oil sample to Schaeffer’s. No metal in oil.

Knock seems a little quieter when cold. The knock is very consistent at idle and low rpms. Seems to go away over 1500 rpm.

Anyone have a similar situation? The diesel shop here is not sure what to do next?
Thanks