Popping when you twist your knees is normal, but I wouldn't do it habitually. The lateral stability of the joint is what's really important - if you're unsure of it, have your doctor check it out for you. Over the long run, Aikido will strengthen your knees and other joints, if you don't abuse them, but getting there you may experience a parade of sore body parts. I went through sore wrists, shoulders, knees, back, hips, ankles, etc. - each took its turn. It was like a chain reaction - when one would recover, the next would take its place as the weak link. That's the message your body is sending you. "Make this part stronger!"
Back to knees. I wear knee pads so both my knees and my gi pants will last longer. Every one has their favorite kind, but I have tried many and have done lots of research, so I'll recommend these:
Anyone I know who uses Trace knee pads (including myself) raves about them being second to none. Check them out at:
http://www.softball.com/softball/pro...roductDesc=100
but they can be had much cheaper at:
http://batterschoice.com/product_inf...roducts_id=780
Note: They are sold individually, so you must order two to get a pair.
Just one more VERY important piece of knee advice - KEEP THEM POINTING THE SAME DIRECTION AS YOUR TOES! If you bend them to the inside of your toes, you risk serious injury.