My left knee is making a crackling sound, any ideas?

It doesn't hurt and I've rested it as well I'm just wondering if anyone else has musical body parts.  =) I've notices that my arms play their own melody occasionally when I do push ups. Again none of this hurts but the sounds can be a tad off-putting as I'm working out.

I have a physical coming up and I'll make sure to bring it up with my doctor. Thanks in advance for your replies.

If there's no pain, don't worry about it. It happens with age. In my classes, some people just creak away, sometimes it makes us all laugh.  I find that the more I move a particular body part, the less it creaks.  I think it's permanent in my knees, though.  :) 

It's when there's pain that you should worry.

That happens to my mother. She has arthritis. I'm not sure about any of this of course but it wouldn't hurt to as your doctor about it.

my ankles and knees creak whenever i take a couple of steps (walking, running, etc.), and it's quite disconcerting for people when they run next to me cause i have 4 different joints creaking in succession...i don't know whether it's something to worry about though. I've had it since i a little kid, and it has never hurt.

My knees and elbows always snap, crackle, and pop like a bowl of Rice Krispies.  They've always done it and it doesn't hurt at all.  I ask my doctor about it when I was about 13 and he said that it wasn't anything to worry about and that some people just pop , lol.  If you're going in for a physical anyway though it's worth asking about.  After all, it's always good to have a second opinion.  If your doctor says their might be something wrong, let me know so I can worry too! ^_^

Thank you all so much for your replies. I will just keep working and get some input from my doctor. Sakuragreenlily, no worrying for either of us. We could always start a marching band.  =)