Why does my car hum when driving and decrease when breaking?

Answer 1

A car will normally hum (or vibrate) while driving or under load, accelerating or climbing a hill because there is damage or wear primarily in the driveline (on a rear wheel drive car) or one of the drive axles (on a front wheel drive car) Often times driveline damage begins at impact with an object that the car runs over, or when being high-centered on a curb or rock. The driver often doesn't realise that anything is amiss, but, sometime later, begins to notice the humming noise emitted by the driveline under load. For every rotation the tires complete, the driveline will turn 3 1/2 to 4 1/4 times so they're really vibrating fast. If it is a front wheel drive car, the drive axles use constant velocity joints on both ends to line up with the transmission housing on the one end, and the wheel and spindle assembly on the other end. These joints are sealed from dirt, sand, rocks, and debris by a simple rubber boot that encases the joint. They can either simply wear out (80 to one hundred thirty thousand miles) or be damaged by heavy snow and ice or simple road debris. Once they crack, they begin to pick up water spraying up off the tire that is laden with sand and debris. The sand and debris wash the grease off the bearing and the bearing fails quite rapidly, causing the axle shaft to oscilate (wobble) making a humming noise.