Every once in a while I'll get a hum and vibration from the steering wheel when turning (very faint and very quick like clearing your throat). At first I was thinking the pump was going out, but no matter how much I try to get it to do it or replicate the circumstances, it still remains 100% random. The fluid was a hillbilly ****-hair low so I topped it off, and no leaks anywhere in the system. My only idea is an air bubble

 

Mine will do that time from time, it always winds up being front tire pressure is low, I keep fronts at 70psi and rear to 80psi. It was getting real bad and found out I only had 50psi up front, I guess a 80 degree temp change makes a difference

 

I had the exact same issue. Turned out it was the rubber boot that is on the steering column. The Ford dealership found it. They removed the boot and the noise disappeared. I'm running around without the boot now; hopefully that's not going to turn into an issue ...