Popping sound when braking

[BigRed]

This problem has been around since I bought my truck about a month ago.

This always happens the first time I brake going forward after I have gone in reverse and hit the brakes. And, it only does it once. It happens as soon as I hit the brakes. It sounds to me that it's coming from the passenger side front brake (but not absolutely sure).

Does anyone have a clue what this could be? I haven't had time to look at the brakes, but I should this week sometime. What should I look for?

Thanks!