You got a cummins, there should be a switch on the dash that says something like eng dia. It is a momentary toggle usually on the far left.
With the truck shut off, hold the switch down, turn the key on and the check engine light should stay lite after the ecm cycles through the checks.
If it has a active code, then it will stop buzzing for a second, then spit out the code. It will be a three digit code with a second pause between each digit, don't release the switch until it stops for at least 5 seconds.
For example -
buzz buzz buzz buzz pause buzz buzz buzz pause buzz buzz buzz buzz
means 434 which is a common one, the ecm voltage is too low, check your cables.
Do you lose power when you are driving?
If so it could be a problem with the fuel pressure sensor.
Did you check your coolant level?