firstly, is the fan you bought an axial fan? If so, it's better to replace it with a centrifugal fan, which can reduce some noise. Secondly, you can install noise elimination equipment outside the fan. If there is an air duct, the air duct and the fan must be connected flexibly to strengthen the fixation of the pipeline. Moreover, the iron sheet of the pipeline should not be too thin to reduce the vibration of the pipeline, so as not to cause more noise. If possible, make another layer of sound insulation cotton on the outside of the pipeline.