Customer has a DD13. They had active code SPN 157 FMI 10 (Undercapacity in high pressure circuit) that was active. The troubleshooting led them to replace the PLV, and this fixed the code. However, the injectors were still making noises.
The client ran the injector tests, ran multiple road & shop idle tests, and it was very random when it would make the noises. Cutting out the cylinders would also lead to random results. They contacted the local Detroit dealer, who said to replace all 6 injectors if they injector hold-downs were torqued right.
The customer question was
Why all 6 injectors? Obviously not a cheap repair.
What is happening is that one injector will form a crack under the copper washer. If you look closely where the copper washer seals against the injector you will find it. This is a known issue with DD engines, and it will often NOT through a fault code or cause injector/cylinder cut-out tests to help you. Another customer had performed all the injector tests, and was sure it was #2. Upon inspection, they found #2 just fine but cylinder #4 was causing the issue.
What happens with this cracked injector is that it allows compression air into the fuel gallery in the head. This then goes back into the fuel filter module, and causes aeration in the fuel system. So now aerated fuel is coming back into the pump, into the fuel rail, and causing the chirp. This same symptom can be seen with missing injector washers, double stacked washers, or even a fuel pump.
This is why Detroit recommends all 6 -- If one is cracked, the likely hood of all of them being cracked or cracking soon is very high.