Vibration After Header Installation

[OnTheThrottle]

Hey guys, I just installed some obx 4-2-1 headers on my 99 altima gxe. They sound great and the engine seems to be running normally. However, when i'm driving and my engine reaches between 2500 and 3000 rpm my car begins to shake pretty loudly and violently. I knew that there would be much more vibration after installing headers but I'm not sure if this is right. I can hear stuff being shaken loose inside my dash! I already checked the exhaust hangers and they're all intact and sturdy. Help!