Squeaking when turning the steering wheel Subaru pillar


I too am one who hears suspicious noises from the VA area.

Vehicle: Impreza Kombi 2.0R, MY2006, km reading 50,000.

These noises have occurred twice so far:

1) On the way home from my Black Forest vacation last year. Outside temperature approx. 30 ° C
These noises occurred after a driving distance of 250 km (the last km of which was country road). It sounded like a deep squeaking noise that was constant. During a stopover, in my opinion, I was able to rule out the shock absorbers / springs.
On the rest of the drive home (about 100 km on the motorway) the noise was still there from time to time.

The next day I drove to the workshop and as it always is, the noise was gone and you couldn't see anything except a slightly damp steering gear.
From that day until today the noise has not occurred.

2) My wife drove to work today, everything ok ... After 5 hours, she drove another 30 km. There the car has been in the sun for 3 hours (approx. 30 ° C).
On the way home, the noise started again and increased more and more up to home, so that it could be heard continuously. She says it was worse on corners than on the straights.
I then drove the vehicle and it sounded like a constant mechanical noise again (not metal on metal), but the same on the straights as in curves, I didn't want to drive more than a small lap in the village with it.
When I was standing, I steered and was able to understand the noise again, but then it sounded like a squeaking noise. The noise is definitely coming from the engine compartment on the driver's side.

I've also read through the following threads, but the sound doesn't come up as it does in these threads:
1) Outback problem
2) clack steering gear

After an initial rough visual inspection, I cannot see any leaks from above in the engine compartment or on the axle and steering sleeves. In the case of an axle boot, I could only see that a little bit of grease had escaped from a clamp, but otherwise the boot is dry.

My suspicions are now directed at the steering / steering gear area (I don't want to think about the costs of replacing the steering gear ), but I have not heard of a defective steering gear with the same noises as above.

Now I need help from you, event. the specialists @Woidl or @Kartoffelbrei have already had something similar in their workshops.

Many thanks in advance,

Greetings, Jens
P.S. I'm going to the workshop on Tuesday, but as I know the Murphy, the noise is gone again.

EDIT: Servo fluid reservoir is full.