The second is spoke windup created during tightening, which produces an artificially correct tension level. When wound-up spokes untwist and release their extra tension, they ping the first time the wheel is ridden. In effect, that's the sound of the wheel going out of true. Windup can be minimized by lubricating the spoke threads and by compensating with the spoke wrench. You can also mark one side of each spoke with a felt-tip pen to see when it turns.
Some builders grip the spokes with smooth-jawed pliers.
If spokes are prevented from winding up, potentially dangerous methods of releasing stress are unnecessary. These include bouncing the wheel on the floor, or laying it on its side, grabbing the rim at 3 and 9 o'clock, and vigorously pushing down. This is a popular technique, but it's best reserved for straightening bent rims, not building new ones. Surviving a massive side load might be a sign of strength, but it might also weaken or ruin a wheel. Now that your wheel is straight, centered, fully tensioned and free of hidden stresses, you might be inclined to slap on tires and hit the road.