For most cars, the source of the sound comes from air in the power steering pump. The air can come from fluid level being too low. This is usually a good indication that there is a leak. The other comes from fluid cavatation around the pump shaft inside the pump. Cavatation just means it makes bubbles. Which it shouldn't. All fluids on an automobile are designed to resist 'shearing'. Shearing is what happens as an object passes through a liquid. In the case of water, think boat propeller. Lots of bubbles and foam come from the prop 'shearing' the water.
Honestly, doing the service has a lot of benefits, but I would be lying if I said it's a guaranteed fix-all. Nothing is a guaranteed fix-all. However, doing the service reduces the likelyhood of part failure and can often eliminate noise from the pump. Also, Run-Rite offers a 240,000 mile warranty on serviced parts! So if we do the service at regular intervals, you will automatically have coverage IF there is a failure! It's a win-win! You could have up to $2000.00 for repairs! Just click on the Run-Rite link for more information! Then schedule your service with us!