The brief
The owner had been hearing a steady hum on the motorway, the kind that builds with road speed and is easy to dismiss as tyre noise at first. But in long sweeping curves it would step up to a growl, then ease off again as the corner unwound.
That last detail is the giveaway. A wheel bearing carries the car's weight as the wheel turns, and when you corner, the load shifts from one side of the car to the other. A worn bearing being loaded up gets louder; the same noise rising and falling with the corner tells you a bearing is on its way out. He brought the A3 in before the hum became a real noise, which is the right time to do it.
The diagnosis
On the lift, we spun each front wheel by hand and felt for it. The driver-side bearing had clear roughness on rotation, the gritty feel of one whose rollers and races have started to wear, and the hub showed a hint of play it should not have. The passenger side spun smooth and quiet.
So the driver-side bearing was the failed one. But the passenger side was the same age, on the same roads, and a bearing that has done the mileage to wear one side is rarely far behind on the other. So the call was to do both front bearings as a pair, rather than be back for the second one in a few months.
The work
Removed the caliper and disc on each side, then pressed out the failed bearings. Pressing a wheel bearing in and out wants the right tooling and the right pressure; forcing it the wrong way damages the new bearing before it ever turns.
Fitted new Audi-spec bearings, each set to the correct preload, because preload matters: too loose and the bearing knocks and wears fast, too tight and it runs hot. Reassembled the hubs and torqued the axle nuts to spec.
Then spun each wheel by hand again to confirm it turned silently and smoothly with no play, before the wheels went back on.
The outcome
The hum gone. No growl in corners, either direction. No play at the hubs.
The A3 went home quiet through the front again. For the owner, the practical win is a quiet, smooth car, plus front bearings that should now go the distance.
And catching it at the hum-and-growl stage rather than letting a bearing fail completely kept the repair to the bearings themselves; a bearing run to destruction can take the hub or the wheel speed sensor with it.