Joints Are Cracking...

My Joints Are Cracking when I exercise... Mostly my shoulders and knees. It doesn't hurt, just pops.

For example when I'm lifting weights, high up, my shoulders pop almost every time I raise my arms. But if I change the angle a little it stops. Some days it does that, others it doesn't.

I heard that it might just be trapped air.. but its really annoying...

Is it bad?

And is there a way to fix it?