09 Impreza Strange Vibration

I have a 2009 Subaru Impreza Outback Sport with 42k miles. I purchased the vehicle about 2 weeks ago, and since the day I got it i noticed a vibrating noise inside. It is not too loud and so it did not bother me at first, but I have become increasingly more and more annoyed with it.
It is constant, as long as the engine in running. It varies in pitch depending on the rpm of the motor, but never goes away.
Does not change with the air conditioner on or off, and does not change with the radio on or off.
It is not engine noise, and is not noticeable outside the vehicle, but it loud enough that it is heard by all in the car.
I took it to the Subaru dealership and they could not pinpoint the noise, and said that the vehicle was sound.
I then had a mechanic friend look at it, and he traced it to under the driver’s side kick panel, but could not pinpoint it further.
Does anyone have any idea what I am looking for?

Get a mechanics stethoscope at Harbor Frieght. You can use it to isolate where the sound is coming from. You can start at the kick panel.