Noisey Lifter

Usually it's a lazy lifter. The valve springs will slowly bleeed a lifter down when the engine sits. Then when you fire it, a lazy lifter won't pump back up fast. If it was me, and it only does it when it's cold and hasn't been run ina while, I'd probably leave it. If you have to you can replace only one. However you will need to perform the cam break in procedure regardless of whether or not it's 1 or 16.