My 2003 gmc yukon pops when I turn deep. I had a mechanic to tighten up all the bolts underneathe my car, but it is still making a popping noise.

It is hard to tell without hearing your noise but you may have a steering shaft making some noise. They are notorious for it. GM came out with an updated version of it. It makes more of a clunking noise and not a popping noise though. It is not unsafe just annoying if that is your problem. They are about $100 and pretty easy to change and only available from a dealer I believe. You may have a ball joint making some noise. You should be able to take it to a reputable alignment or suspension shop and they should be able to check out your ball joints and suspension parts for you.