First I must explain that the whistling stops when the hot tap is turned on because the diverter valve switches the unit from central heating to domestic hot water heating
with that said, the whistling only occuring with the central heating tells me it is not a combustion issue as if it was constant during burner operation I would say it is either a gas valve or air leak on a gasket within the combustion chamber...since it is only with the CH then I would be more inclined to say it is with the system circulator or an debris within the piping of the CH. Therefore I would recommend a commerical cleaning agent flush. There are commercial boiler cleaning products that get installed into the piping, run for 48hrs, and then flushed out.
let me back up and rephrase something..."I would say its either a gas valve (issue with gas pressure)
Since it has happened since its commissioning may also mean that there was debris within the lines or the pump speed setting is too high. Either way, I would consider a second opinion engineer to check out the problem for you given the information I had provided.
Many thanks for your sage words. There probably is some debris in the pipes as I can hear it rattling sometimes. That said, I wouldn't be surprised if the pump speed is too high as the flow noise is somewhat greatdr than the old boiler and
tank. I will contact a gas safe plumber armed with you advice. Many thanks.