Answers

Tthe squeaking sounds you hear are probably from movement of the boards, either against each other or against the sub-floor beneath the hardwood.  The nails used to install the original sub-floor may have barely missed the joists and are now rubbing against the raw wood.  The popping could be sudden movement of the ajoining hardwood boards. 

You may be able to top nail the hardwood in the joints between the boards.  A more effective solution would be to screw up through the sub-floor into the hardwood.  This will suck the hardwood down tightly against the sub-floor.  Take the opportunity to pull out  any visable nails that could be rubbing against joists. 

The second solution depends on access to the area below your squeaks.

If basement ceiling is unfinished or has drop ceiling you can usually run 1 1/4" screws from underneath.  It is best to have a helper finding the squeaks and try to measure from the outside walls in order to find proper placement of the screws.  Top nailing the boards is a temporary fix at best.