I live in Phoenix and have central air. At times I hear a "water rushing" noise coming from the attic when the A/C is on. It sounds like someone is turning on and off very rapidly a faucet. I just had the A/C serviced and my unit is fine, I have clean filters and there is not any water collecting in the pan in the attic. What could this be?

It is probably the refrigerant in the lines causing the noise. Usually a normal sound but since you just had it serviced I would request a free call from them to check it out.