Is it a ticking noise, or a knocking noise? If it sounds like what rod knock or lifter tick sounds like, more than likely it is the timing chain tensioner.
It has to do with the building of oil pressure, as stated above. It is a common problem for DOHC cars, since it has to build oil pressure from the oil pan to the top of the motor (where the cam and lifters are located), instead of where the cam sits in a pushrod motor (middle to bottom of the motor). There is a TSB for it, GM will replace it once (at the owner's discretion), the problem will come back, and you will be told that it is normal. At least, that was what happened to me over a year ago. It will not happen if the car has been warmed up, and this noise will go away as soon as the car has run for about 5 seconds. My car has been doing it all it's life and seems to have no ill effects. Good luck with the fix.
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