It repeats itself in a certain time interval but it does swing a bit sometimes. If to describe it’s movement in a more straightforward way, it’s like [ on-off-on-off-… ]. While it’s “on”, that’s when you hear it sound which is composed of two parts, one is growing and one is releasing. During the growing phase, you hear something like what you hear when you’re sucking air out of a straw but with thicker texture, along with some bubbly elements. The overall feeling is that it’s going up, and becoming stronger, sort of like gathering energy but in quite a short time like around 0.5 second. The releasing part gives a feeling of going down, from the all-powered status down to zero. The “off” is the time interval between two repeating events.