You won't get any evaporation over time. If you crawl underneath the car, you'll probably find an accumalation of oily dirt around the seals of your rack and pinion system. The leak would be so slow it wouldn't leave a puddle.
Be careful not to overfill your system, as this will destroy it for certain. And as for your "Whirring" sound, this does happen as those pumps age. If the steering doesn't become hard, I wouldn't worry about it. But eventually everything fails. Just keep the fluid at the proper level and if you ever see a leak, get it fixed before it gets worse and costs you more.
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Sunday, February 6th, 2011 AT 10:12 PM