I recently replaced my old Saginaw power steering pump with a new rebuilt AC Delco unit (which was advertised as having the Saginaw-style reservoir, but didn't). When I drive the car, the power steering seems normal, and the new pump doesn't leak, but it makes quite a bit of noise (especially at start-up). It's not a squealing type of noise, but one that sounds more mechanical (at first, I mistook it for loud lifter or rocker noise). I can hear it with the hood up, but it isn't loud enough to hear over the exhaust (which is pretty loud) when driving.
Any ideas on the cause?