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.
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835