definetly see an ortho...
you may have loose ligaments or something... in fact it could be anything.. and there seriously isn't a reason to aggravate something which is already hurting by working out.. see what the good ol' doc has to say... take it from there...
my knee pops when i turn also.. but mine doesn't hurt at all... i got it checked out by a doc and he said that as long as there is no pain its quite normal... so the pain in your case is a good signal that something is wrong...

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**I'm not a veteran!!! Don't Take Advice From Me!!**