2008 Mercury Mariner Hybrid Popping Sound

Popping sounds aren't related to power steering fluid. The most common cause of this symptom is binding upper strut mounts. Reach over the top of a front tire, wrap your fingertips around part of the coil spring, then have a helper turn the steering wheel left and right. You should feel the spring rotate smoothly with the wheel. If the upper mount is binding, you'll feel the spring wind up and build up tension, then suddenly pop free and turn.

This is not exactly a safety issue, but if you find this is not the cause of the noise, the true cause very likely is. Ford has more trouble with steering and suspension parts breaking leading to loss of control and crashes than all other manufacturers combined. Noise and clunks must never be ignored. Those systems should be inspected at a tire and alignment shop at least once a year, and immediately when a new noise shows up.