How do I make my shoes stop squeaking?

I have a pair of North Face hiking boots that i have worn for several years. Since I live and hike in a warm climate, I usually only wear them in the colder months.

This year they seem to have developed an annoying squeak in them. When I walk it goes “Zip, Zop, Zip, Zop, Zip, Zop” (you get the idea). I have never noticed it before this year.
Is the leather too dry? Too old?
Anyone got any suggestions for how to solve this dilemma? If so, and if it works, I will file a new post under the good things that have happened today.