The brief
The owner of this A4 had been living with a low droning hum that came up with road speed and got louder over coarse tarmac. Lately it had a harder, grindy edge to it, and on a long sweeping bend it would change, fading on one steering input and growing on the other.
That last part is the tell. A wheel bearing carries more load on the outside wheel through a corner, so a worn one gets louder when it is loaded and quieter when it is not. The humming that tracks road speed rather than engine speed points the same way. Left alone a dry bearing only gets rougher, chews into the tyre on that corner, and in the worst case lets the wheel move where it should not.
The diagnosis
On the lift each wheel was spun and rocked by hand. One front bearing had the gritty, notchy feel of a bearing on its way out and a faint roar when spun, while the others turned clean and silent. A short road test with the noise rising and falling exactly as that wheel loaded and unloaded confirmed it.
On this A4 the front bearing comes as a unit with the hub, so the fix is to replace the whole hub-and-bearing assembly rather than try to press a loose bearing into a tired housing. That also means the new bearing arrives set to the right preload from the factory, with nothing left to guess.
The work
The wheel came off, then the brake caliper and carrier were hung aside and the disc removed. The hub assembly was unbolted from the upright, the ABS sensor wiring freed, and the old unit drawn out.
The new Audi-spec hub-and-bearing unit went in, bolted up to torque, and the ABS sensor reconnected and seated so the wheel-speed signal reads cleanly. The brakes were rebuilt onto it, the wheel torqued down, and the car taken back out to check the noise was gone and the ABS light stayed off.
The outcome
A quiet corner again, no hum building with speed, and no change through the bends. The ABS light stayed out and the wheel-speed reading was steady.
The A4 went home with the rumble gone and the handling back to normal, and with the worn bearing out before it could ruin the tyre on that corner or do anything worse. A hub unit fitted properly to torque should outlast plenty more sets of tyres.