Unfortunately, that's just the core problem with high-performance blenders like Vitamix or Ninja - these machines are loud because they have to run at high speeds to actually get you that creamy nut butter. If noise really matters to you, you could switch to something like a Blendtec (it's marginally quieter, but not dramatically better) or alternatively combine a good immersion blender with a container - that's way quieter, but it takes more time and effort for thicker stuff. Honestly, if it's just smoothies, cheaper models with better sound insulation would do the job, but for real nut butter you've gotta accept a few decibels no matter what.