Annoying knocking sound...

[xasthur]

This past weekend I planned on installing my U/L control arms. I was only able to install the left lower arm thinking I had enough time but didn't. Well I put the sway bar back on and made sure everything was snug tight. I greased up the arm and took it out for a drive. I kept hearing this knocking thunking type sound from the left rear, but I could only hear it when i was slowing down. It kinda sounded like something was flapping against something, not really sure how to describe it. So I pull it back in my garage and make sure I didn't miss anything. Everything is tightened down. It's been making this thunking sound for the past few days and I cannot figure out wtf it is. Does having 3 stock control arms and 1 aftermarket have anything to do with it? I can't figure it out. Any help would be appreciated. Thanks