Can you get parts for it?
Before a full rebuild take a look at a couple of things:
Check for a water restriction in any way- a partially open ball valve or clogged strainer
Look for a water leak on the inlet side of the pump, in other words Any leak from the water source to the pump inlet. While the pump is running the water leak become a place where air could get in the inlet stream, causing pulsation
Swap regulators with another pump and see if the problem stays with the pump or moves with the regulator.
Check the pump valves for debris or a bad valve. I would caution you to make sure you can still get valves for this pump before you do this. You may remove a damaged valve and not be able to get it back into the pump, basically exacerbating the situation.