Steering noise


Yesterday, as I drove onto an on-ramp (a right hand semi-circle), I noticed that there was a vibrating or buzzing sound that correlated with the degree of the turn–a higher pitch if the turn was sharper…lower for a less severe turn. The off-ramp was a left hand curve, and I noticed no such sound. I’ve also noticed that the steering wheel shimmies slightly when I am driving on the highway. Recently all of the fluid levels were checked, but does this sound like an issue related to the power steering pump, or possibly the tie rods? The car is a 1998 Lincoln Mark VIII.