I recently installed a York PM9 gas-fired system in my house. CFM / BTU was closely matched to the existing Friedrick Oil-fired unit that it replaced. As the new is gas fired, it had the 2" vent and 2" combustion air connections. The vent went outside and the intake was left inside.
The system worked almost flawless. Once the blower kicked up to high speed, a distinct rumble could be heard. It was at such a low pitch that it resonated throughout the entire house. I would check the system for level, good. Check the plenums for vibration, nothing. I was at a loss. Then other day, I was getting the wood stove ready when I listened to the combustion air intake. It was quite loud, then the system began to rumble at high speed. I put my hand partially over the air intake and the rumble went away!
I ended up putting a 2" x 1-1/4" reducer in the inlet and now the system runs at high speed with no rumble at all. And the combustion flame appears to be happier as well (was blue with slight orange/red), now almost all blue).
I am thinking that the design of the system included a design of so many feet of 2" combustion air pipe (assuming that the majority of systems would draw air from the outside). Since I am drawing from the inside, the system does not see that static, so I had to create it.
Now all is well.