I live in northern Illinois and over the last winter, my 2006 CTS started making a whoooooing sound when it's cold. It goes away after it warms up. It doesn't appear to be related to the engine RPM, but it happens when I decelerate and the tranny downshifts. Transmission fluid is at 50%. When should it be changed, since I can't see any dipstick to check it? Wife said the car stopped funny a couple days ago, said she had to press real hard on the brakes to stop. Could the noise be coming from the booster on the master cylinder? Looking for things to check.