the speed line is famous for the sound they make when they hit the ground, you have nothing to worry about, swing away
Mine did the exact same thing about a week after I got it in May. I continued to use it all year and it hits really well. Here is a video I posted back in May. It sounds a little worse now, but like it said hits great still.
http://www.youtube.com/watch?v=3Z-uyleKb_s