It sounds like internal pieces may be worn excessively or damaged due to running dry.
inside the pump there is what is called the ring, rotor and vanes. There are also two plates that they are sandwiched between. Usually what happens is that someone ran the pump out of fluid and damaged (galled the surfaces of the ring or the plates. If it chatters when you turn the wheel the rotor is broken. I have only seen that a couple of times but it does happen.
They aren't that hard to rebuild and parts are usually available.
The parts would be a ring, rotor and vane kit, the two plates and a seal kit.
One note, if anyone used a hammer to remove or replace the original pulley they may have damaged the rotor at that time.