While setting up the plane from cold+dark, my computer fans, hence noise, get increasingly louder as I progress through the Checklist.

Is there any trick I can quickly toggle or do to keep my room quiet until I'm ready to pushback?

nVidia and XP12 btw, but other computer setups could apply too.

My first step is to see if fans are doing more RPM than needed. Keep a close watch on temperatures and adjust the response curves in the BIOS or in an external program (or your graphics card driver program).

I try to have them turn as fast as I just can't hear them yet even for lower temperatures to prevent heat build-up.

Watch airflow. Is it too warm in your computer case? If they turn increasingly quicker like in your case that might be the case (pun intended). Is it better if you keep the case open? Maybe an additional case fan helps or a new case with good cooling.

Always go for the biggest fans – they are more quiet because they turn slower to shuffle the same amount of air. And 4 fans at low RPM make less noise than 1 on high RPM.

You can always set an FPS cap, using something like RTSS if you have an nVidia card where you can assign a hotkey to toggle it on and off. If you set the FPS cap 10-20% below the FPS you normally get while setting up your flight in the cockpit and toggle it on as soon as you load in, your system will be under less load hence cooler and hence quieter and you'll barely notice the FPS drop in this phase of flight. When you're ready to pushback, toggle your FPS cap off and away you go!