Updates after years of investigation:
Eventually most of time (90%?) the whining is gone. Things I have tried that might have helped: replacing power steering reservoir, flushing/bleeding power steering fluid, transfer case fluid, new diverter valves, new spark plugs/coils and transmission fluid service. I'd certainly try power steering fluid flush/bleeding first -- the cost is low anyway. For my case it seems what made the biggest difference is the transmission service (fluid and filter). However I am still puzzled given that the whining noise didn't go away immediately after the service but took about 2~3 weeks to get the whining attenuated -- might be it takes time for new fluid to cycle through the whole system but it is hard to imagine for 2~3 weeks. Anyway I think it is recommended to service the transmission at 60K miles or so although Porsche says 160K. The uncertainty is also the recent heat wave hitting socal -- maybe whining goes away simply because of higher ambient temperature? While I think unlikely I will wait till it cools down to call victory.
