Power Steering reservoir and fluid help

[ChrisUK]

Hi,

I have a late 2004 X5 E53 3.0D and I've noticed that occasionally after driving for half a minute down the road and brake I get a groany kind of noise. I thought it might have been something in the engine not fully pumped etc due to be cold but I cant remember ever hearing this sound in the past 3 years of ownership!

I wondered if it could be something to do with the power steering reservoir. I looked online and the reservoir I seem to see all the time has a different cap to mine?! Mine does say ATF on it and its located near the front of the engine near the coolent cap.

When I unscrewed the cap with ATF written on it I can see that the fluid is bearly touching the bottom of the ajoined small plastic dipstick level.

Once Ive heard the noise as described above it may happen once more but more often that not it never comes back until the following day after the car has been stood for some time. It doesnt do this when engine is warm. Braking and steering doesnt seem to be effected to be honest. The car has 94k on the clock and has been serviced by BMW but no idea as to when the power steering fluid was last changed.

My questions are;
1) I am looking at the correct reservoir arent I (it has ATF on the lid)?

2) Can I use Castrol Transmax DEX III Multivehicle ATF fluid as a topup, drain/refresh? Or does it need to be simply Castrol DEX III?

3) Could this be the problem?

Thanks for your help,
Chris