01 T&C clapping metal sound from R-rear

It's like something is loose but I have taken the tire off and checked everywhere. Seems more noticable on dirt roads. Anywhere from 10 to 30 mph. I have checked it in motion and all I can see is that the rear axle is moving up and down fast, like a fast clap but with a metal clapping sound added to it. Seems like its traveling up and down to fast at 10 mph, I dont know though. Anyone have any ideas? Thanks