are you sure is a valve noise?? or is a piston slap?
Wear or improper adjustment.
The fuel injectors make the ticking noise you hear at startup, especially when the engine is cold (colder temperatures).
does it have a ecotek valve fitted?
check the cadillidic converter under the car.
first off is the clacking noise when in park as well? if it is it may be a valve tap. you can research a valve tap it is common and usually not a huge problem right away.
There could be several reasons why you have a noisy hydraulic valve lifter. One reason could be worn our parts in the entire valve train. A loose rocker arm could also cause noise.
or it could be worn valve stem seals. which causes a little bit of oil to build up on the valves when you shut the engine off or stopped at idol. this is burnt offon startup or when you takeoff.
a broken rod or valve lifter
idle air control valve
If clicking while trying to start then might be a bad starter. If clicking while running, then might be valve/lifter noise.
You didn't mention whether you hear it all the time or just on startup? If on startup, the timing chain has a tendancy of making noise especially when cold. Usually the lifters don't make noise in these cars unless the oil is really dirty or oil level really low.
All turbos will give a blow off valve noise but the higher the pressure in the boost pipes (eg) higher boost, the louder the blow off noise and if you have a recirculation valve remove that with a remap and the noise will be even louder.