Owners of 2016-2018 Cadillac CTS-V or 2015-2018 Escalade vehicles may note a rattle or ticking noise that is louder inside the vehicle. The noise may be picked up at the back of the left valve cover. The noise may also sound like a lifter tick or a fuel line buzz or ringing noise.
Listen to the fuel line connection at the left side of the engine. If the noise is heard best at the fuel line a if using a stethoscope, the fuel feed line between the high pressure fuel pump and the chassis fuel line could be the cause, allowing high pressure pulsation back to the low side of the fuel system. There is not a good way to test the check valve, so the line will need to be replaced. There are two different fuel line types that contain this check valve and can vary by model. On the LT4 CTS-V this line will have a jumper line with quick disconnects at both ends.