power steering fluid level issues

What is your question?

For about the last month I have to add about 4-5 0z. of fluid every week,depending on how much I drive. When system makes groaning noises and I check it and it`s low with moderate air bubbles. I re-fill it to the proper level and cycle the system side to side lightly and the noise goes away. What`s puzzling is there are no visible leaks anywhere! Where`s all that fluid going?