Sound used for decay is the same as attacks.
This is probably more of an oversight than a true bug, but Prime provides audio feedback for attacks and provides audio feedback for decay.
But the audio feedback for natural daily decay, sounds like someone attacking your portals. Decay does not cause explosions. Decay does not cause mods to die. Decay should not sound like an attack.
My suggestion would be something like the De-ath Star Shields powering down sound.