I've been out of practice and out of shape for a few years. Lately I've been working out with a punching bag (heavy, suspended), but I've noticed that my arms make a snapping/cracking noise when I throw straight punches. I'm fairly certain it's coming from my shoulder area. There is no pain, but it worries me a little. Is this normal? Is it a symptom of not warming up enough? having bad form? I'd appreciate any advice.