Noisy power steering when warm

Hi Guys,
I recently acquired a 2005 Jimny and have been slowly fixing up its numerous issues, often with help from the responses in this forum so thank you! Currently a problem I’m trying to deal with is that the power steering pump is making a terrible sound when it’s warm. I feel like it’s the sound of a hydraulic pressure relief or something like that, but it makes the sound even when the wheel isn’t turning. If I turn on the aircon it seems to subside slightly. The sound starts at around 2000 rpm and is cleanly audible over the engine when driving. The actual steering functions fine. 
I have changed the fluid twice and even replaced the pump with another one (from a wrecker/breaker) and it’s still making the noise. Only the pressure switch was swapped from the old pump. I’m wondering if this normal, or did I manage to buy a second dud pump, or is there something in the circuit causing the issue? Has anyone else experienced this sound? 

The other question I have is whether the top of your power steering reservoir is glued or not? I can remove the fill cap but there is a mesh filter under that which prevents me from getting to the fluid or even seeing it. The main lid is solidly attached and any attempt to remove it has been totally unsuccessful. This makes flushing the fluid a lot messier than It should be…