Creaky Staircase

the stairs would have moved and the timber dried out in time , so you can buy/make sone small wedges to reduce the creaking , drive them under the step treads , this will fix the problem.
Is it the whole staircase or just the treads that squeak? If it's the treads sprinkle with talcum powder (even if carpeted) and jump up and down on them. Talcum powder goes between the gaps. Works a treat.