Just thought I'd post this as maybe someone might have the same problem.
My 2003 156 2.5 V6 had an extremely loud noise coming from the power steering pump on start-up. Same sound as though there's no oil in the system (even though there was). The sound would persist for about 30 seconds or until the car heated just a little (Only when cold) then the noise would go away. Starting it again or later that day, the sound would not be there. Inspecting the oil, it was black-ish, and there were no leaks or kinks in the pipes. I checked the spec in the booklet and bought 2L of the specified power steering fluid. It costed like a R100. I went home and replaced the fluid in a matter of 30 min.
All easy, just pop off the return line at the resevoir, block off the pipe at the resevoir so that the oil within the resevoir does not leak out. Connect a long hose to the pipe now disconnected and have it run to a container so that all the old oil can be bled out. This can get a little messy but sure is sure as hell very easy. Fill the resevoir up with fresh oil and start the car. Now be careful, the oil is gonna deplete quite quickly so as soon as you have started the car, head to the front and fill up the resevoir (Best to open all the bottles in advance so that you can just pour). After flushing 1L, switch off the engine, fill up the resevoir and manually turn the steering wheel a couple of turns left to right. Doubt this is necesary, just made me feel a little better. Ha Ha, then refit the pipes as they came off, try not to spill and top up as necesary.
I did this last night, started her up this morning and it was rather cold... No noise, very quiet, working perfectly... My Conclusion, at 180 000km oil does not have the same lubricity and has to be changed. Doubt it was ever changed by the dealers. So at startup the oil is too cold to offer sufficient lubrication, only once up to temperature.