I appreciate where you are coming from, but I do really get exasperated at the number of posts asking for advice on a DIY forum which get the immediate answer, "call an engineer". I have fitted a new central heating water pump, replaced burner jets, and added a magnetic filter to my heating system. I have bled the complete system and I have tore apart the burner and rebuilt it and I have serviced the oil pump. I have also carried out numerous other plumbing jobs myself without having to call in an engineer, because most jobs are not difficult if you are logical and decent at doing home repair work.
I am fully aware of the risks with pressurised systems, but I like to think I have a reasonable level of common sense, and I know my own limitations. I know where to draw the line. The risk of failure or explosion from an unvented cylinder is extremely remote and would take a catastrophic failure of several safety systems before such an event would happen. In any event, I cannot see how unbolting an EV and bolting on an exact same EV would increase the risk at all. Even manufacturers such as Kingspan have instructional videos on Youtube explaining how to change an EV, its hardly difficult.
On checking, droning and whining from the hot water system is a fairly regular occurrence, and all I am trying to do at the moment is get an idea for what I may have missed. I am sure this is an easy fix. There was no drone from the old EV installation, so I am fairly sure there is no valve failure. But if it comes to that ultimately, I would get an engineer in to check and replace a safety valve. I will however do what I can myself first with the help of advice from here, and what I can source elsewhere, which I have always done when faced with issues around the house.