Ban news: using the CPU will generate heat. Up to 15 Watts (well, 25, but 15 on average). It will heat up the case and the fan will run.
As a "laptop" it's pretty much a non-issue for me as it never gets too warm to touch/hold/lift and with the design of the stand and the power in/behind the screen it's more comfortable that a traditional laptop (where the bottom gets warm/hot). As a desktop machine, at full throttle, the fan is about as loud as the air vent from my Mitsubishi heater (which is pretty darned quiet) and the same volume as my old Dell i7-920 tower at idle (and quieter than when the old Dell is crunching numbers).
While my 1st SP4 was loud and hot a lot of time, my 2nd one is literally silent. I've heard the fan come on once, during the firmware update from 11/2, but even using Office and VS 2015 it has stayed quiet. Very impressed, especially compared to SP3.
I installed GTA3 from the windows store to see if I could get the fan to come on. It came on after around 15 minutes and was as loud as the SP3 but the device itself wasn't as warm. So it looks like the fan will come on under heavy use but it's less frequent and cooler than the SP3.
Pain in the arse is an understatement. You will certainly be cutting a whole in the case and, in all likelihood, be destroying the machine in the attempt if you're off in your cuts' marking or depth by more than a millimeter or two.