noise in engine and poor gas mileage

When purchased as CPO in 2013 car ran quiet and smooth. Within a month, engine started hesitating and rattling. Took to dealer multiple times and was told that they needed to replace fuel pump. They also replaced a chain (no I don't remember which chain). Every time I took it in for an oil change I talked about the hesitation and noise and was told it was normal. Took car in today as part of oil consumption test and asked about the hesitation and noise and was told to purchase their kit for gas and that should fix hesitation. Something about the water in ethanol in gas. Also said that noise might be the fuel pump lifter (?). I told him that they had replaced the fuel pump in 2013. He didn't reply.