The ticking noise is probably the main control solenoid on the high-pressure fuel pump. Like other direct-injection engines, the 3.6-liter in the CTS uses a low-pressure pump to send fuel from the gas tank to the engine and a second, high-pressure pump to send fuel to the injectors. In GM’s DI V-6, this pump is mechanically driven by the camshaft. The reason for the noise level is the extremely high pressures at which the direct-injection fuel system runs. While a port-injection system runs at about 40 psi, GM’s direct-injection system can run as high as 1740 psi. Turning a high-pressure flow like that on and off makes noise in a couple of ways. First, there’s a “water hammer” effect, such as when you abruptly turn off a sink faucet and then hear noises reverberate through the pipes in your house, only with a lot more force over a shorter period of time. Second, the solenoids that control the fuel flow, both at the high-pressure pump and in the injectors, need to be much stronger in a DI engine and are consequently noisier.