2003 CTS Rear noise.
 

I have this horrible "humming" noise coming from my rear. It's not the wheels cause the noise doesn't stop when stopped or sitting at idle. It is a constant "humming". I can only find info about the differentials but that doesn't seem like my problem.

Possibly the electric fuel pump? It is in the gas tank, towards the back of the car. Open the gas door and remove the filler cap and listen for the noise.