niose that increases with speed from the rear of my 2012 Buick La Crosse
Have you ever replaced the tires in the rear? If you hear a roar, it may be tires. If not, very likely that is the problem and they need to be replaced even if you see good tread. Swap them to the front and see if the noise follows.
Is it a lower pitched growl? Likely a wheel bearing. Get it checked ASAP!
Scrape-ing type noise? Likely a brake problem. Get it checked ASAP!