I don't have a lot of experience with this...I help out with a local church, and they are meeting in a fellowship hall while renovating their sanctuary, and have set up a temporary sound booth. I am attempting to record the services but having trouble. We are using a Behringer board, and the RCA jacks from the "Tape Out" section are going to the 3.5mm line in jack on my 2007 MacBook Pro. However, as soon as I plug the cord into the line input, even if the other end of the cord isn't plugged in, I hear a rhythmic popping noise that registers on the audio meters, whether it's recording or not. This shows up on the audio meters in Garageband AND the line input meter in System Preferences. I've tried multiple cords with the same problem. The popping starts as soon as I plug it in and doesn't stop until I unplug it. Any thoughts? Thanks very much!