Noisy flue pipe

I have a flue pipe running through a chase in the corner of my bedroom that makes knocking noises every time the boiler turns on, loud enough to wake us at night. This is in a three story row house. There is a gas boiler (175k BTU) for heat, and two water heater tanks, located in the cellar — all three are connected to the same exhaust flue pipe, which runs straight up to the roof through all three stories above. I believe the flue pipe is 8-inch B-vent galvanized steel. The heating system...

Noisy flue pipe