My '06 LaCrosse has 19,076 miles as of today. My concern with a noise(low grade rumble) that takes place when I back up (reverse), and the noise only lasts a couple seconds at most, and it stops. I don't hear or feel anything after this only noise. My thoughts are, it has something to do with the reverse, or in the brake system. The dealer here in my area are noted for their tearing apart a car and making it worse. The transmission shifts and works fine as is, and the fluid is normal/