This is simply writing the triangle period with custom code that's not based on vblank timings (at least not 60Hz) but rather taking more CPU usage. Ibelieve Sunsoft got their croaking sound in the Fester's quest and Journey to Silius game over screens in a similar fashion.
The Sunsoft croaks I believe are just using the triangle's "linear counter" feature which can make very short triangle bursts (240Hz width) that Famitracker can't do normally. I don't think it's CPU driven, still on vsync, just using an obscure hardware feature.
Now I'm all kinds of confused because I don't understand what FM or AM are, so I'll just assume these morphing triangle sounds are the work of a Lovecraftian deity
I don't think DPCM would work because I could only have a constant sine or triangle LFO pattern versus longer periods of successive, unchanged samples due to the nature of delta encoding.
It's not exactly what I want, and I'm still figuring out some things, but here's an attempt using DPCM:
