Noritz dual (radiant and domestic) tankless system over pressurized

We have a Noritz NRCB199DV dual tankless system in a new construction with Warmboard radiant heat. When we occupied during the summer of 2018 with the radiant heat turned off we noticed nothing unusual. However, since we began to use radiant heat beginning in mid-September, we have noticed loud noises coming from the unit when it begins to produce hot water (either domestic or radiant) and the pressure relief valve is triggered and water is expelled. In response to this, the plumbing company that installed the system bled air from the unit and the radiant heat system which lowered the psi from about 45 to 25-30 generally (max should be about 22 psi I am told). However, the noise and over pressuring continues, particularly it would seem after the unit has been idle for a while, and the psi may increase to 30-35 after the noise/pressure relief. The plumbing company insists that it bled all the air from the water lines, since to me it seems that air in the lines may still be the problem, and that air may rise and accumulate in the unit if it has been idle for a while. It has been four months now and neither the plumber or Noritz with whom he consults seems to have an answer. The plumber asked me this week about psi for water coming into the house from the municipal supply...the municipality came out and measured 52-60 psi. The plumbing company also asked about hammer arrestors, and although they are on the washing machine, I do not believe they contribute to the problem. It seems that neither the plumber/installer nor Noritz have an answer. I would ask the plumber to try bleeding the air from the system again or determining if there is a way that air is re-entering. Does anyone have any ideas or suggestions? Thanks!