That could be it Delta. With all this furnace and boiler talk, I didnt think they have a Zone damper system, and still dont know if they do. If thats the case, good read Delta.. It may even be the bypass damper if they have one.
Originally posted by DeltaT
You probably have a zone damper slamming shut or open in that zone that has been disconnected and the noise stops. And the 6 hour intervals might be due to the timer for fresh air that will cause the blower in the furnace to energize, the outside air damper to open, and all the 3 zone dampers to open (which would explain the noise)for total air flow throughout your home.
Have the tech run each zone damper open and closed to see if that is the noise.