Doc...my point was that I shouldn't be able to hear any furnace operations through the registers for my particular furnace. However, with less "quiet" furnaces, of course you should be able to hear this as the nature of duct work being like a primitive phone line, will transmit sound quite effectively. I used to play a game with my kids where I would speak into a gap in one of my supply ducts in the basement (opened to run some cables), and they could hear me clear as a bell from a 2nd floor register. This is of course in addition to duct-induced noises.
Originally posted by docholiday
The furnace has nothing to do with the sound of air coming out of the registers, thats a duct design issue. Each register has a designed velocity and volume intended, the ducts (provided the furnace is proper) provide volume and velocity. Poor ductwork can make any system noisy or ineffective.