So after I have been driving for say 5-10 minutes and I pull up to a red light I hear a knocking sound very similar to if someone knocks with thier knuckles on a 2 by 4. The sound happens every 8-15 seconds while idle. It sounds like it's coming from behind the passenger seat underneath. I see to only hear it at night but that's most likely because it is quite then. When I pull away no sound at all. Any input would be loved.

Thanks,
Nick