power steering system making noise

If the power steering fluid level was low, the pump would suck in air, and that will cause a buzzing noise that gets louder when turning the steering wheel. It can take some time for the air to be fully expelled and the noise to stop, but you need to determine where the fluid is being lost. Until the leak is found and repaired, there is little point in continuing to add fluid.

So my next step is to determined where the fluid is leak and how do I find that?

You have to look for it. Common places to find leakage are at the crimped fittings on the high-pressure hose, and the internal seals on the rack and pinion steering gear. Those usually leak fluid into the rubber accordion boots covering the inner tie rod ends. At first you need to remove the smaller clamp on either boot, then use a hooked tool to pull the boot open. If fluid is leaking from either side, it will travel through the small metal tube connecting both sides, and will fill both boots. Eventually, once both boots become full, the fluid will be forced out and will make a puddle on the ground.