High Pitched Humming Noise--'97 2500

I have somewhat of a "high pitched" humming noise coming from the middle of the truck. I hear it every immediately upon starting the car and it is not intermittent. It makes the noise constantly. I am not sure if it is the exhaust system, but I am trying to eliminate all possibilities before I take it in. Any thoughts? Thanks in advance!

If it is come from around the gas tank, it's probably the fuel pump making noise.

replace your fuel pressure regulator
and problem solved -:)