After I reverse, whether in a straight line or turned, I heard one pop on the front passenger side after putting the gear in drive. I haven't checked the passenger yet to look for anything. It sounds like it has something to do with the transmission. I got this transmission replaced about 3 years ago. Any suggestions before I start looking? I have a 2006 Trailblazer 4x4.

 

it could be a lot of things ... suspension, driveline, engine/tranny mounts ....
You'll just have to inspect them all for looseness.