OK, that's a bit of important information .... you are using a pumped accumulator running @ ~ 2bar. Is it an ST?
As a first, the engineer should be looking to align the pre-charge closer to the input pressure to minimise the differential once the cylinder is fully heated. It sounds like there is a 'sweet spot' with the EV where the pressure release through the EV's baffle is causing a resonance, creating the 'fog horn' noises. You did say that a new PRV/Control group was installed? That would generally rule out the PRV. It may also be an idea to calibrated the PRV closer to the Accumulator's output.
I'm afraid to say though, you could really do with an engineer that is experienced with working with these types of systems, especially when yours is a bit of a hybrid, using an accumulator. This is really UV/EV 1'o'1 and he should be running through all of this stuff. That being said, Gledhill should also be asking the right questions and you shouldn't really have to rely on a forum to get to the bottom of it all for you, not that giving you advice is any issue it's just a long drawn out process for you and it shouldn't need to be.
You really need someone that can work through all of this at one visit, rather than having to come back here once something is tried but doesn't work.