Charge is fine, was 3 bar (incorrectly left by installing engineers), has now been set to 2.8 in accordance with manufacturer's instructions. What makes you say EV? Some people saying EV, some people saying definitely not EV. Would love to know what the reasoning behind each answer is. Would whatever issue you suspect the EV has cause a couple of seconds of what sounds like a spring twanging?
Having a read about check valve chatter it seems that turbulence can be a cause, and elbows close to the valve can cause that. Looking at combo valve design and the 4th picture I posted, is it possible that the increased pressure/flow from the EV emptying is causing turbulence in the combo where the check valve is located and causing that to chatter?