[M-A8]

Funny you mentioned both issues as I am currently dealing with the power steering issue and creaking noise is exactly same as on your car.
This was happening on the A8 I've just recently sold and is happening on S8 as well...but...only in cold temperatures as you both guys said. It drives me nuts.

1781cc, do you think fitting new rubbers on top of the mount could solve this issue which is possibly caused by the age and wear??

When my upper arms were on the way out I've had almost identical issue, but it was getting worst and worst with the temperature rising.

My steering pump started making noise recently and because of the reservoir location you can't just undo the cap and check the level on regular basis, I've decided last weekend to remove top art of the air box and ducting to check the fluid level.
And yeah, my fluid level was below minimum so that got my attention straight away.
After removing the passenger side fog light grill I have noticed that both hoses by the intercooler have a slight leak pass the hose clips. One is from the bottom of the reservoir to the intercooler and second one from the steering rack to the intercooler.
10min ago I finished talking to my mate who has an Audi garage and RS4s have almost identical layout for the power steering system and they suffer from the same issue. This can be very quickly solved as it has been tested on many cars already by putting another (second) clip on each hose. Or replacing the hose with new clips but if the hoses are ok then its a bit of a pointless job.
What my plan of action is for tomorrow, I'm waiting for the steering fluid to arrive, I'll be fitting an extra hose clip on each hose, clean it all with brake cleaner, topping up the fluid to max and observe and listen for the noise to go away.