2001 Jeep Cherokee Knocking from rear passenger side at low

I just bought a 2001 jeep cherokee sport, V6 4WD. There is a loud knocking/rattle when I am stopped at traffic lights coming from the rear passenger side. My first thought was the muffler or exhaust but it doesn't appear to be loose. However, what is odd is that if I stop abruptly I do not have the noise or if I put it in neutral the same. It only does it when its in drive. Any ideas on how to remedy this obnoxious noise? Any feed back would be appreciated