2006 Cummins engine knock high RPM

I have a 2006 9400 I with a 435 Cummins ISX. Between RPMs of 1400 and 1650 the engine has a noise that is a mix of a knocking and popping sound during acceleration only. It is slightly worse under a load.
There is no loss of power. I have changed the oil, oil filter and fuel filter also searched for any leaks in the exhaust system.
The only other thing that may be related is the truck is only getting 4.75 miles per gallon.
This is a new truck to me that I have only put 8K miles on. It was part of a package deal with two other good running trucks. Overall the truck has plenty of power and runs great other than the knocking noise and poor fuel mileage. Any help would be greatly appreciated thank you in advance.

 

Would pulling the injectors at a idle be able to find a bad one if it is over fueling?

 

Okay, I'll get it heat on and try that out first. I'm on a tight budget for this truck until my motor rebuild is done on my Detroit so I'm trying my best to self-diagnose

 

It is a cpl 8287 isx450st. Ill try to sign up for the cummins quick serve. Will the free edition provide the procedure?