Disco 2 Loud Rattle/knocking from ACE pump?

I've got a Disco 2 v8 that I recently replaced the power steering and ACE pump on after a major mechanical failure ripped the ACE wheel off the pump and destroyed the belt. After replacement all was fine for a couple days then started experiencing a very loud rattle that got worse and worse. I brought it to a mechanic who said he replaced the filter within the reservoir? Which I didn't know was possible. Was fine for 50miles but it just started back up with a vengeance. It is louder than the engine and clearly, something is wrong. I have checked all bolts are secured and cannot find any external faults regarding the sound. I have recorded a video in case someone has an idea. My best guess is that the ace is sucking air instead of fluid? Have searched the forums extensively without much luck in a solution.

Wow that’s horrendous. Id say that pump is not happy. Air in the system doesnt do that! Its pumping but would say the movement in the res is that great, but hard to see. Check for play on the pulley with belt off on the pump. Is it getting hot in the pump? They dont like running dry fir very long before burning up. If its a used one, it could just be shagged.