I recently bought tires for my 05 Z71 and while it was in I had it aligned. A few weeks later, I noticed this sound that kept getting more persistant. I checked under the truck and did not see anything suspisous but that did not satisify me so I took it back in and we found that the nut was working itself off the upper A-Arm on the passenger side. The guy at the shop was nice enough to realign the truck for free and made sure that the nut was tightened down. Today, after driving it for about 100 miles, I noticed when I make a sharp turn, the sound is back, so I checked the nut again. It was backing itself off, not much but enough that I could tighten it back with my hand.

Does anyone have any idea what would cause this? I know it was tightened up the second time because I watched the guy tighten it.