Please explain the reason behind the following: the vibration material used to separate the plenum and the furnace, secondly, the enlarged 4 " diameter baffle on the exhaust vent.
The vibration material used on the plenum is to help reduce noise transmission through the duct work, there should be one on the return air duct drop as well. A stainless steel flexible gas line is also used on the pulse for the same reason
The enlarged 4" diameter baffle is a muffler, again to help reduce noise. Some units would have them on both the exhaust vent and the intake vent.