I mean I'm playing BG2 through (well, the whole series). I'm getting towards the end of Throne of Bhaal.

And trying to get through the final battles is making an almighty whooshing noise come from the back of the computer. Some friends seem to think it's the graphics card fan, but I cannot think why: my computer is a nearly-new Windows 10 model with a 4 GB graphics card, and the specs / requirements for BG2 officially say that it needs only a 4 MB (!!!). It never did this on my old (Windows XP) computer from 2008: in fact it never even did this on my previous computer before THAT, also a Windows XP rig from 2002.

No matter what is happening in-game, my modern computer should be able to cope with it - whether it's the graphics card or the processor, or WHATEVER. Hell, given the difference between the requirements and the actual capabilities, my computer shouldn't even notice that it's running. (Not running anything else at the time, either, no.)

So what might be causing this? It starts after a few minutes of playing: and then winds down back to normal nearly-silent again over the course of a few minutes after I stop.

It could be a mis-aligned fan, which has nothing to do with capabilities. You need to open up the case and very carefully check for alignment, seating and obstructions, not forgetting fluff.

That's possible.

Another possibility, that several friends have come up with (and given that this seems to specifically happen only on older games) is that it's one of those games which was set to require 100% of the CPU no matter that modern-day CPUs are a whole lot more powerful than they were ? If that's the case, what would I do about it?

That's not the case. Take a look at the attached image. That image was taken while I had two games running (Drakensang and Baldur's Gate 2 in ToB, 12 tabs open in Firefox, plus various other small utilities and applications. The highest CPU (which I have assigned dedicated use for PhysX) core was at 49%, and the GPU load was negligible, with all of those things running.