1968 Pontiac Tempest Car Vibration and noise

Car has 119,000 miles on it.

After returning from a trip out on the highway (130 miles at around 65 miles per hour) I noticed a faint noise and slight vibration in my car when I got back to town and slowed to speeds around 25-30 mph. I've had a mechanic with experience working on vintage cars look at it and he thinks the problem is with the power steering pump or the steering box. The first thing we tried was replacing the power steering pump. The noise and vibration are still there. The noise appears at speeds of 25-30 mph (slowing down or speeding up). I can feel a slight vibration in the steering wheel at 25-30 mph also. The old power steering pump had metal shavings in the lines and the mechanic wants to flush the power steering lines before we move on to other possible causes. The mechanic has checked other possible causes (wheel bearings, suspension components, U joints, etc. But still thinks the problem is the power steering pump or steering box). Any ideas what could be causing this problem?