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Hi,
Yes, my theory is also that the actuator is worn.
It is much harder to feel the play in the actuator when you put the car on jacks and try to rock the tires. You have to be 2 persons to check it this way. The car on jacks, one person rocking the tire and one person under the car feeling the play in the actuator with the hand.
Better is to be under the car when the car is resting on the tires with the suspension in normal height. Just put the car on ramps (or wooden blocks) a little higher than normal just enough to be able to get under the rear from back. Grab where the track rod is fixed to the actuator and rock it hard up and down (not "in and out" from the actuator). My play is inside the rubber "hose".
I have not checked the play with the engine running.
I think that if the ECU is faulty , then you would have a car not running straight when the steering wheel is 12 o'clock (misalignment). Or the rear steering would be locked in position zero all the time. I do not think the rumbling comes from a faulty ECU, it has to be mechanical.
The play is not "inside" the actuator where the steering motor is forcing the actuator back and forth. The play is in "bearings" of the connectors that . If you rock the left side connector you do not feel any movement in the right side connector. This means that if I "lock" the actuator in zero position, the problem will still be there. This means also that the tires do not turn that much because of the play in the actuator (since it is not in the direction "in-out" of the actuator).
The actuator in 760 is the same for all F01/F02 including 730 diesel with less than half of the power. Maybe the actuator is too weak to handle the power in 760? Part no is 33346776294, you find used actuators on ebay.
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