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There isn't any direct setting on the Valliant to adjust the fan speed. There are settings in the engineering controls that allow you to limit the boiler load. By adjusting the load settings lower the fan operating speed should automatically adjust to match - The d0 parameter which is described as 'Adjustable heating part load in KW' is the primary engineering setting for boiler load - If, for example, d0 is currently set to 15 you could try lowering the setting to 14 or 13 to test the resulting fan speed and noise. I am not 100% sure how, procedurally, to get to the setting as I don't have a Valliant Combi to experiment with right now, but there should be install/setup instructions in the installation manuals which would guide you to the engineering menu and settings.
I suppose a second option would be to call the service back and explain the situation - They may make a return trip for free (We normally would) to make more adjustments and maybe even show you how to access the settings.