Hi guys.

Sometimes when I stop the car for some minutes and let the engine run, the P/S (I think) starts to beep single beeps. But as soon as I drive away it stops. (P/S not reliably anyway). I already replaced the control unit with no effect. The pump maybe?

Harry

 

Get rid of the power steering, you can "feel" the car much better Plus its not harder to drive at all, i think its easier

 

If it's an S4 it's worth fixing.
Check the condition of power steering belt, as slipping belts cause erratic steering problems.
The power steering computer has it's own error code system which will emit gentle beeps (versus louder the redline, LOW COOLANT, LOW OIL buzzer) when it encounters a problem with the power steering system.
1-beep, pause, repeat - in case abnormality of speed sensor signal:
In case broken wires or short circuits occur to sensors or harnesses, the buzzer sounds 60-seconds after the engine speed signal of 2200 rpm continues for 18-seconds.
2-beeps, pause, repeat - in case abnormality of engine speed signal:
The buzzer sounds when the vehicle speed signal is input and no engine speed signal is input (short circuit, broken wire, etc.)
3-beeps, pause, repeat - in case abnormality of stepping motor signal:
The buzzer sounds when there are broken wires or short circuits in coils or harnesses.
4-beeps, pause, repeat - in case abnormality of the power steering microcomputer:
The buzzer sounds when there are circuit troubles other than with the micro computer.
5-beeps, pause, repeat - in case abnormality of steering sensor signal:
The buzzer sounds 60-seconds after broken wire or short circuit occurs to sensors or harnesses when the vehicle speed is above 40Km/h (25 mph).
Continuous beep - in case abnormality of the power steering microcomputer:
The buzzer sounds if the microcomputer becomes abnormal and cannot control the system.