A loud hum is hard to pinpoint. Could be the power steering pump is out of fluid. If it's been out of fluid for a few weeks it's probably shot by now. It was probably leaking from a power steering hose initially. I will need more info to better help you. How many miles does your car have? Over a 100,000? Is the check engine light on? Are there any changes in the frequency (pitch) of the noise when you do certain things? eg. press on the brakes, turn the steering wheel, etc. How well maintained is this vehicle? eg. regular oil changes, maintainence items, etc. ?