The brief
The A4 had a disturbing sound, a hum or growl that built up the faster it went and changed pitch when the car was turning. Disturbing sounds can come from a lot of places on a car, so a proper diagnosis comes first, and that noise pattern is the classic worn-wheel-bearing tell. He brought it in. The wheel bearing lets the wheel spin freely on its hub while carrying the car's weight and the cornering loads. It's a sealed unit, and over the miles the bearing wears, the rollers and races pit, and it gets rough and noisy, the hum that rises with speed. Because cornering shifts the load from one side of the car to the other, the noise changes as you turn, which is how you tell which corner it is. A worn bearing only gets rougher, and a badly worn one can damage the hub and the brakes, so it needs replacing.
The diagnosis
On the lift the wheels got spun and rocked by hand. The bearing on the noisy corner was rough turning and had a trace of play, exactly what makes that growl, while the other three spun smooth and tight. The noise tracked to that one corner. That's a bearing replacement on the affected corner, it's a sealed unit, you don't repack it, so the call was a complete bearing or hub assembly as the design calls for, pressed and torqued to spec.
The work
The corner was stripped down, the worn wheel bearing pressed out, and a new genuine Audi-spec bearing fitted, everything cleaned up and reassembled with the hub nut and the suspension fasteners torqued to the manual figures. The wheel was refitted and spun to confirm it turned free and silent. A road test confirmed the hum and growl were gone across the speed range and through corners both ways.
The outcome
A quiet, smooth-running corner, no hum, no growl, the noise gone whether straight or turning. The A4 went home with the bearing sorted. A worn wheel bearing only gets rougher and can take the hub and brakes with it if it's left, so changing it on the affected corner put it right before it became a bigger job.
