I have a new NIBE pump (S735) that replaced my old, also NIBE (Fighter 640P) pump. It's a bit more silent than the older one usually, but sometimes it starts what sounds like a jet engine which is pretty loud. I can hear it from upstairs, far away from the pump, coming from the vents. I am not sure what it's doing when it makes that noise but it made me a bit upset with it, as initially I had hoped for a quieter house, and the pump was pretty expensive!