Residential 2psi natural gas service. Gas comes into a manifold where the line regulator dumbs it down to 9 inches wc before going out to the furnace and hot water heater (only gas appliances). Recently noticed that when nothing is calling for gas, the post-regulator pressure creeps up over time. 9, 9.5, 10, 10.5, 11, etc. Once something calls for gas, pressure goes back down to 8.5. Gas stops flowing, goes to 9.5...then here comes the creep. First thought was the line regulator was dying so I replaced it. Brand new regulator and...NO CHANGE! Same phenomenon. For the record, the incoming 2psi service is rock solid -- no changes there at all. I'm stumped. Anyone have any ideas? Static pressure? Thermal expansion of the gas in the manifold, maybe from the furnace? Something wrong with either the furnace or hot water heater? This is a normal thing and I've just never noticed it before? Something else?Thanks for all ideas!