Yeah, its not an instruction anywhere but instead a unofficial shortcut that some, including myself, use. It does take considerable leverage against the exhaust crossover to allow enough room to clear the pans, but it doesn't strain it any more than normal driveline torqueing of the exhaust pipe and components.
And there aren't any "bellows". There are solid plastic doors the actuators move, which largely seal just by themselves, and then have a strip of foam around the edge to better seal the door. The foam does deteriorate over time, but this is unlikely to be a significant cause of the problem.
are the heater controls set for hot or cold air?
If it's set for hot, and say, to get air from outside the cab (vs recirculated air), even w the fan off, air will get pushed through the hvac system, over the heater core and into the cab. Switching it to cold will reduce the temp to whatever it is outside. Switching to recirculate mode (which may mean switching to vents or whatever, read your owners manual), will reduce the amount of airflow through the hvac system.
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