Experience values for the sound level in dB_A of a central controlled residential ventilation system

With a series of measures (dispensing with a pollen filter in the bypass, extending the silencer from 42 cm to 100 cm, an extra silencer for the line to the loudest room, sound insulation volume elements (SVE) in two rooms, turning down the system) the noise situation could ultimately be significantly improved, so that the system is still clearly audible, but no longer extremely loud.

However, I still have the suspicion that all these measures were only necessary because the noise development of the system in the basement is already 12 dB_A (=4 times as loud) above the manufacturer's specification, which surprises me somewhat and which was also acknowledged during the first on-site visit that the system is significantly louder than other systems.

100cm mufflers are actually normal ... did it at least run under warranty?

Yes, the extension of the silencer to 100 cm was the subsequent restoration of the target condition.

The general contractor did not charge us for any of the mentioned measures. He had assured us DIN 4109 in the construction contract, which means a maximum of 35 dB_A in the rooms.