No one likes a squeaky floor, especially when stealth is of the essence, such as as during the night when you feel the need to raid the refrigerator. When the subfloor is concrete and the boards are glued to it, the squeaking is happening because the glue bond has weakened, allowing the boards to lift and rub against one another. There is an easy way to temporarily stop the squeaks, but if you want a more permanent solution, you have to replace the failed glue so the boards no longer move.
Mark the squeaks by walking along each floorboard and laying masking tape whenever you hear one. The tape will give you a visual indication of subfloor problems. For example, a predominance of squeaks on one area of the floor indicates a subfloor depression or moisture problem. A uniform distribution of tape, on the on the other hand, suggests wood movements as the cause of the squeaking.
Spread talc or graphite powder on the floor and brush it into the joints between floorboards with a paintbrush to lubricate the joints and address squeaking over the entire floor. Test the floor, and when the squeaks disappear, vacuum the excess powder off the floor. You may have to repeat this treatment periodically to control the squeaking.
Fortify the glue under a specific board or group of boards by injecting glue with a syringe. You can buy kits that include the syringe and either water-based adhesive -- intended for engineered flooring -- and solvent based for solid and bamboo flooring.
Drill holes to get the glue under the floorboards. Use a 1/8-inch drill bit or snip the tip off of a 6d finish nail with wire snippers and use it as a drill bit. If you use a kit, drill the hole with the bit that comes in the kit. You'll need at least one hole in the vicinity of each squeak.
Draw adhesive into the syringe, then insert the tip into one of the holes you drilled and squeeze glue into the hole. Tap on the board while you inject the glue, and keeping injecting as long as you hear a hollow sound. If you can't detect a hollow sound at all, inject glue until it starts backing out of the hole.
Remove the syringe, wipe off the excess glue with a rag moistened with mineral spirits or, if you use a kit, use the wipers supplied with it.
Fill the hole with floor filler, or, if you use a kit, with the dowels supplied. If you use dowels, you may need to color them with stain or felt-tip pen. You don't need to weigh down the floorboard; simply let the glue set, and it should prevent the board from moving.