The brief
The A4 had developed a rumble or growl that built up the faster it went, the kind of low drone you start straining to hear over and can't unhear once you've noticed it. He brought it in. That noise pattern is the classic worn-wheel-bearing tell. 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 rumble that rises with speed. A worn bearing only gets rougher, and a badly worn one can develop play and start damaging the hub and the brakes, so it needs replacing before it does.
The diagnosis
On the lift the wheels got spun and rocked by hand. The bearing on the noisy corner was rough turning, gritty under load, 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 rumble and growl were gone across the speed range and through corners both ways.
The outcome
A quiet, smooth-running corner, no rumble, no growl, the drone 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.