Audi A4 - ABS Sensor?
 

I've just brought my mates Audi of him Audi 2.5 tdi quattro (diesel) Now then when you turn the car on there is a beep sound which comes on and I'm told by my mate it's the abs sensor? He said that this sound would fail in the MOT test but can be fixed anyone able to help me out shred some light on why it's beeping what these sensors are and where i can buy some new ones as I'm told they'll need replacing!! Thnx :)

By the way v reg 1999