I have an 02 Cavalier with 156K. A while ago now, I noticed a slight hum coming out of the front wheel area when the steering wheel is turned slightly to the left. It started at highway speeds and has creeped down to about 40 mph. The brakes were due for a replacement, so I got new pads and the rotors machined. BUT, the hum continues. Any ideas?
My original post failed to note you saying this was happening at highway speeds, so it became irrelevant.
Does this happen ONLY when the wheel is turned to the left…not the right as well?
Checked the P.S. fluid lately? How about the hoses? Worst case scenario, (~gULp~), Rack-and-Pinion.
I don’t know what you call a hum, but my one-ton ford work truck will hum slightly while turning. I think it’s a new feature letting you know your power steering is still working. Or your car is trying to hum out a Carpenter’s song for you.
In the meantime, try to map out straight routes, to avoid turning at all. Pulling the e-brake around a corner minimalizes the need to turn the wheel very far. 
The hum is ONLY when the wheel is turned to the left, not the right. And, it’s not just left turns when it hums but even slightly angled to left of center. Checked the powering steering fluid and it was just below the cold line (when it was cold) on my slightly angled driveway. The fluid was reddish-orange, thin and watery (not thick like oil) and smelled slightly of exhaust.