Noise in applications such as the Audi A4 and A6 that are not resolved once the pump has been changed and the air has been purged and left to settle out of the oil can be due to the deterioration of the high pressure line from the pump to the steering rack and pinion.
The internal liner og the high pressure hose can become delaminated from the outer reinforcement layer and the effect is that as fluid passed through the tube it creates a reed effect (like a saxaphone) the vibrating tube creates audible sound waves.
The fluid in the steering system is an excellent noise transporter and so it can be very hard to pinpoint the origin of the noise in the system. Infact it can sound like you know where the sound comes from but it is not actually the source of the vibration causing the noise as it has traveled away from source.
The only solution is to change the high pressure line from the rack to the pump.
