I have a newer second story home built in 2007. When people walk upstairs it cracks and pops and seems to go across the whole floor very loud. There are no nail pops or out of the ordinary settling cracks. What causes this and how can it be fixed?

It could be a sheet metal header or block off for your hvac system. Sometimes they can be tight and if the floor joists flex a bit when walked upon the sound of the metal popping can be very annoying. If it happens when walking near a second floor return
That is my guess. If you can pin point it you could hop or stomp on the spot and many times it will crease the metal just enough to stop the noise.

It could also be the nails that hold down the subfloor. This is what "glue and screw" is supposed to prevent although glue and ringshanks work fine for me. Tough to fix if the finished floor is down because its best to find and pull the offending nails. Ive heard that they can be fixed with a special screw through carpet if you can find the floor framing layout and tighten things up near the offending nails.

Takes some experience but there is a different sound from nails and sheet metal. Sheet metal generally has a lower frequency.