Why do you hear a valve tap when you start your car when its cold?

Because the oil has settled to the bottom of engine, colder weather causes oil to thicken,and it takes a minute to circulate back up to the rest of the engine,causing a light lifter tapping sound. let it idle for a few minutes before driving.

If it persists after warm-up, you may need new lifters (and, possibly, a new cam) or an oil additive, although you need to consider the age of the engine, and the dealer's recommendations. I would not try to adjust lifters (if they are adjustable, which most are not), unless you have adjustable lifters and you can prove that the adjustment has drifted due to a loose nut. You can prove that by checking the torque on the nut and the range of quietness in the adjustment - one full turn is the normal range of adjustment - if you have a narrow range, put the adjustment back where it was. Please, consult the shop manual or a qualified mechanic before starting something like this.