Over the past couple of months I have noticed a popping noise in the front end of my 1999 GMC Sonoma 4WD. I will try and explain where the noise is coming from and when I have heard it. I initially started hearing this noise when I would back out of my driveway each morning. It would make a popping noise within just a few feet of reversing. I could drive the 5 miles to work and reverse into my parking spot and never hear the noise after the initial few feet of reversing. It does not make a noise when going forward. I tried backing into the garage one night and the next morning when I went to work, there was no noise moving forward. A couple of weeks ago I tried another test. I started my truck and before I even put it in gear, I turned the wheel to the right and then to the left. I tried this test several times as the first time it made a popping noise when I got the wheel turned all the way to the right. The next morning I turned it to the left first and it made a popping noise almost immediately. I have performed the circular driving test for CV joints and there is no noise. I have replaced the sway bar bushings and end links and have torqued the end links to 14 ft-lbs. I have had the ball joints, both upper and lower, changed in the last 6 months as well as both wheel bearings changed in the past 18 months (the most recent was 4 months ago). I just had a pre trip safety inspection at Les Schwabb and they said brakes and struts look good. Sorry this was so long, but I'm at a loss for what the cause might be. Any help would be greatly appreciated. Thanks in advance.