Re: No pain, but tendon \"pops\" back and forth under elbow.
It might be due to an imbalance in muscular development in your arms. You said it happens when you are half-way through a curling motion? Maybe one part of your triceps is not as strong as the other muscles in your arm and so it isn't anchored as well (not as much tension). Sometimes these things are just structural abnormalities that people have that cause things like that to happen. If it starts to bother you, see a doctor (physical therapist), but if you make sure all of your arm muscles are proportional in strength, you may be able to stop it from happening.
Hope this helps.