For mpg my first thought was intake.. i built a custom intake for the car with spare parts and my avg went from 17.7mpg to 20.1... i'd say the 60 bucks i spent for the air straightener and cork sport dry flow is definately worth it.
Its your evap system running self test.. the buzzing noises and such are from your tank/canister checking pressures. in my ten years of the automotive world ive had maybe 2 ppl say something about it but its been there since they have had the vsv's