Loud humming

Does it do this while in park?

In park, revving the engine?

Turning the wheel?

Listen to your power steering pump, it's possible that you are low on fluid. Or if it was low, and you added fluid, air might be in the system (aerated). Letting it sit awhile, may let the air bubbles rise out in the reservoir. Hopefully it's not shelling out!

A "Cheap", rather "Free", "Hands On", diagnoses, might be that knowledgeable guy, down at the auto parts store.

The Medic