You can clean the power steering pump cap, and check the level on the stick attached to the underside of the cap. You can trace the lines from the pump and check for excessive residue for prevalent leaks. Faulty alternators give off a whining noise when turning, you can have a load test performed at your local auto parts store if you choose as well. Wheel bearings give a low drowning hum too, so make sure to perform process of elimination.
Check the power steering pump for fluid, it's marked on the side of the bottle with a line for full. If you have a leak there will most likely be fluid on the ground. You might have a certified technician check it out for you.
〉 Answered on Aug 26th, 2008 by Donna Burmeister, Service Manager at Classic Nissan