My left knee squeaks, too, and it's from doing step before I realized that it was very important to keep one's legs in good alignment - doing Susan Powter in the early 90's!
Don't let the knees skew out to one side or the other, especially when doing step. The squeak still continues if I'm not careful, but keeping my knees in good alignment with my ankles and the rest of the leg helps considerably.
I find that when doing leg presses, it is very easy for my knee to skew out to the side (either way), too.
The squeak is not painful, but it is disconcerting.