Cadillac Repair Questions? Ask a Mechanic for Answers ASAP
The first step is to confirm that it's a noise from the transmission, and not the rear axle. This is easier on a lift, with someone in the car and the wheels spinning in gear.
A whining noise from the trans can be considered normal, depending on how loud it is, what gears or speeds it does it, etc. Any check engine light or anything like that going on with the car?
How loud is it, is it obnoxious or just there? Does the pitch change when the transmission shifts gears, or does it go up and down with vehicle speed, regardless of what gear it's in? Does it make a difference if you're accelerating or decelerating? If you shift the car into neutral at 30mph, does the noise change?