cracking ankles/knees&hips during exercise...

It's not actually your bone cracking, just a release of gas bubbles from the fluid surrounding your joints.

Still unnerving, but not dangerous.

Not always, I thought this too but since during activity I was finding that my knees started to get hot and a sore I went to the dr and found out that it was actually cartilage pieces broken off inside of my knees. http://orthopedics.about.com/cs/patelladisorders/a/chondromalacia.htm