Cadillac Repair: My 2002 sts starting making a whirring, o2 sensor, fuel pump

Question
Coming from the  outside rear of the vehicle. It not related to the movement of
the car. Its worse whe its cold and seems to go away when warmed up. Also
when I turn on my rear window defrost it changes tone. I had an O2 sensor go
bad and it kinda reminds me of this.....

Answer
Sounds like a an electric motor but other than the fuel pump, I can't think of anything at the rear of the car that would do that. Unfortunately, a thing like this really needs to have an experienced mechanic take a listen. In most cases, they can give a quick diagnosis.