Often times an upstairs wall is not securely attached to the floor, especially inside walls and when you walk by them they sqeak or crackle or pop. Your problem could very easily be the need of gluing down the footer and using lag bolts into the trusses. The sound is from very slight movement of the wall in relation to the floor. Oh yea, a stud finder is the best way to find a stud without making holes in the drywall. One way to secure the wall to the floor without removing the drywall is to use a rotozip and cut below the trim line and screw the wall down then just replace the trim (note keep the removed strip of drywall to reuse as spacers under the trim when replaced.