Not sure anyone can help you much with the humming, since one would probably have to ride in your car to figure out exactly where it is coming from.
In any case, just a couple of comments on previous posts in this thread:
* Regarding the humming sound: I have heard an electronic humming coming from the hybrid electronics at around 40-45 mph. It is only noticeable when the weather is nice and I'm driving with the windows down. I suspect that this sound is always present, but only noticeable under the right conditions. If that is what you hear, there probably isn't anything you can do about it other than roll the windows up and turn on the blower for the heater or air conditioner.
The humming could also come from the tread design of your tires. Do you have snow tires or ones with large/deep aggressive sipes? Again, I'd recommend trading wheels and tires with another Prius for a test drive.
* Regarding poor fuel economy: Do you track your mileage with every fillup? What mileage were you getting during the same period last year? Mileage is really affected by tire pressure, driving style and outside temperature (which affects heater and air conditioner use). I have Michelin HydroEdge tires rated at 44 psi and keep them inflated to 44/42. My daily commute is on city surface streets at 40-45 mph, with synchronized traffic lights. Fuel economy is highest at 51-53 mpg in the spring and fall when I drive with the windows down without the heater or air conditioner running. It is the lowest at 44-46 mpg in the summer and winter when both are running most of the time. Also, I drive almost exclusively with the cruise control on to maximize the percentage of time that the hybrid system is controlling acceleration, cruising and braking, and I use the throttle and brakes only during full-throttle starts from every traffic light and the brakes only during the last few feet of slowing. And, I try to time the synchronized traffic lights so I do not stop at intersections. This sounds counter-intuitive, but this technique has proven itself to be the most efficient over the years.