While this could be a number of things. At first I was thinking maybe the vent pipe had came apart or is coming apart causing the nosie but after reason your post further and sound went away then started back once ac was turned on is werid?
Are you sure it's not the furance itself rattling? Regardless if gas company can't figure out, I would call your hvac service provider and have then out to look at the system and see if they can make since of the noise and offer solutions to correct.