I'm at a loss with my car just now. Have already had rear bearing changed, two new tyres fitted and noise still persists. Started as a 'hum' at 40mph which I reckoned was a rear wheel bearing. Before going on holiday I got the rear N/S bearing changed. £240 While on holiday it started to make a 'wah wah' noise. I lived with it thinking it could be road noise, and have heard and read information on tyre rub. Rear tyres were 2-3mm and needed replaced, but the wear was consistent across the tyre surface. Put the car back to the same German car specialist that did the rear bearing. They checked the rear diff oil level (was changed at 115k by local mechanic), which had a little visible dampness, but no leak. Oil level was fine and they listened to it too. Said bearings were fine. Replaced a couple of exhaust plates and charged me a half hour labour. £80 Moved front tyres (5-6mm) to rear and replaced fronts with Dunlop Fast Response 205/55/16V tyres. Car instantly felt better and I thought I'd solved the problem. £180 Car is 2001 (AKN) 2.5 TDI quattro avant sport and has been serviced religiously by myself since 69k with no expense spared, but my patience is running out. Now at 147k. Timing belt, water-pump and auxilliary belts changed last Autumn. Just renewed road tax for 6 months. £150 Noise is still there. could be rear diff, bearing, central prop carrier or front diff? i can also feel a little vibration through accelerator. What else can I do? Yes, sending it to a main dealer would be peace of mind. Convinced its quattro related, but my heart (sharp intake of breath from Audi reception) or my wallet can't take another financial hit if its the rear diff, etc. Could the German specialist have goosed the bearing. Have read even pros can screw up fitting a bearing if its not flush before using hydraulic press? Any suggestions welcome.