My left knee gives out some times when walking down hill. he doesn't really hurt but sometimes pops. should i be concerned?

Yes: Every time your knee give out you have to be concerned, also when your knee pop you shoud be concern.Please check it out with orthopedic surgeon. Be carefull.Thank.

Slight concerned: You seem concerned enough that you posed the question. Because of the popping and giving out you should have your knee evaluated by a sport's doc or ortho surgeon. From you symptoms i would question some patella subluxation and quad weakness. B i cannot rule out some other mechanical such as a torn meniscus. Your symptoms may be resolvable with some physical therapy.

Vague: It might be weak and need some strengthening. Both knees, actually. I would see an orthopedic Dr. and be checked just to be sure.