Got new a/c unit. The drain is gravity fed out of the a-coil box and drains outside the house. It constantly makes a clicking and gurgling sound draining water. There is no restriction and water drains fine. What causes the noise and how do I stop it?

Install a trap in the drain line.

I agree completely with hvac1000 (nice to hear from ya). Yep install a running trap in the horizontal prortion of the drain. The noise is from air blowing through the drain. A trap will eliminate this.