Check the hanger/hangers and make sure there not to tight, when I got my duals put on the idiot hung them to tight and they rattled for a month. After six times on the rack at the exhaust shop the old man that runs the place finally looked at it and to the nitwit that installed them they were to tight, he loosened them up and havent had a problem since. Needless to say I'm not going back there 
Anyway good luck.
Anyway good luck.
