My Noctua NH-D15 G2 keeps switching between 600-800 RPM during dialogue, causing a hum. Fix?

While trying to enjoy the story in Kai no Kiseki, my Noctua NH-D15 G2 started making this faint but incredibly annoying low-frequency drone. It happened whenever the CPU hovered between 45℃ and 52℃, causing the fans to bounce between two speed steps and creating physical resonance. Looking at the curve, the default stepped adjustment was way too coarse. I first tried locking the fans at 1000 RPM; the hum stopped, but it felt wasteful and I could still hear the wind shear in a dead-silent room. I switched to a smooth curve method, breaking the temperature range into 5℃ increments and adding a 3-second response delay to stop the instant jumping. Using a decibel meter, the ambient noise dropped from 35dB to 28dB - 30dB, which is a massive subjective improvement. I actually accidentally inverted the curve during setup, which almost let the CPU hit 80℃ because the fans slowed down as it got hotter. Now cores stay between 55℃ - 62℃. Verified everything with real-time noise logs.