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.