It seems to be coming from the rear seat area immediately behind the passenger seat. Bugging the hell outta me. It seems to have appeared after they replaced the rear quarter light! I hope it isn't behind the panelling!
For me the rear seat belt retractor mechanism was rattling. I just removed the plastic cover plate and stuffed some foam in their to brace the retractor. I don't intend to use the rear belts anyway. Put some tension on the rear belts to see if that's it.