Update.
Well I have to say, laughable though it may seem, the complaint is justified. If one was to compare a typical loud trailer exhausting with a DAF or Scania horn then this is like a Class 37 locomotive horn. The thing screams not just makes a loud hissing noise. The annoying thing is consistently replicating it. Two trailers exhibit the fault and although I have not experienced it myself with another unit I am told it does it on both units when the unit handbrake is released. On the unit I have had coupled up the red line pressure does not always cut out at exactly the same pressure, but only by a matter of 0.1 or 0.2 bar. It is nudging the higher end of the demanded trailer operating pressure, but the noise is not happening at this pressure, it seems to be more reliably at around the pressure when I assume the air processor bobbin cuts in to recharge the unit’s trailer reservoir. The trailer’s own tank is well within its specified pressure range when the noise occurs. I am not convinced that this is a unit issue . If I maintained the unit I would be checking the airdrier filter, but that is down to the R&M dealer to do. Since all sound is a factor of vibration I am more of the opinion that a certain pressure is critical for the diaphragm in the Modulator QRV. What I would like to try if I was continuing with the unit is adjusting the APU bobbin.