Only when the motor is running do I hear this faint buzzing. Almost like the faint sound of a weedwacker from far away. When I put my head behind the passenger seat in the trunk portion with the doors closed can i hear it.

Also when i put my head near my passenger rear tire i think i can also hear it. Hard to pinpoint with the exhaust right there...

What kind of pump is back there that may be running?

 

Fuel pump in gas tank.

You'll probably also here it when you turn the key to "on" without starting the car.

 

Subscribed. I have a hissing (don't know if its the same thing you're hearing). Very faint, from behind the passenger seat. Sounds like static from the radio but it's there even when the radio is off.

 

That's the thing. I don't hear it when the ignition is on. I get the feeling it's something with my breaks. Lately my E break light has been turning on without the E break being engaged.
And sometimes when I pull the Ebreak I can feel a quick sudden vibration.