in addition, I have found that stringers sometimes squeak when the bottom stringer is not settled on a cement block or haydoc block. Sometimes I have seen decks with the pre cut stringers and each one just resting on the bare ground. When you walk on them, ground settling, the stringers are not in tandem and not level and all even. I like to create a pylon if you will and have each stringer resting on something that is wide footed and in line with all other stringers. This will help.
Then to, are all your treads "sucked up" to the stringers? Meaning, did you fully screw the screws in so that the two boards meet and fit tight?
Did you put a stringer every two feet? If you span them too far, then you get a bounce and a squeak.