Actually, the cooler weather the last couple of weeks in the northeast have caused higher start up idle and longer warmups which will account for the loss of gas mileage. I can tell my current tank is off compared to the last one. Also, if you're making more power, you tend to use it more at first. Right now you probably have a heavy foot because it's more fun to drive. I wouldn't change anything until you go through several tanks of gas but see if you can isolate where the rattling comes from.