Why does the sound suddenly disappear when the Mi Smart Kettle Pro is heated to keep warm, and the temperature rises slower?

In order to take into account the time and accuracy of heat preservation, Mi Smart Kettle Pro adopts two-stage heat preservation. Take the normal temperature of 20°C and the heat preservation block of 70°C as an example: the first stage: 20~65°C, using full power Heating, the temperature rises rapidly to 65°C; due to high-power heating, a loud sound can be heard. The second paragraph: 65-70°C, using low-power heating, gradually climbed from 65°C to 70°C, and then maintained At this temperature; due to switching to low-power heating, the temperature climb slows down and the sound is almost inaudible.