Hoverfly Episyrphus balteatus “warm-up”!

Last morning I was looking for some call or song of migratory birds that in this season migrate through our latitudes, when I was attracted, in audio headphones, by the buzz of an insect, a particularly “annoying” buzz. I directed my Sennheiser ME66 half shotgun directional microphone towards the sound source, by means of hearing, without first seeing what and what the subject was. Once the sound was focused, I noticed that it was emitted by a Hoverfly sp. laid on an Elderberry leaf.
That seemed unusual to me in that I had never noticed this behavior by the hoverflies, but I must say the truth that it is not a group of insects to which I have ever paid particular attention.
At first I thought of something inherent to a territorial / reproductive behavior.
I took some photos, made some short videos, but above all I made some audio recordings. Talking to my friend Cesare Brizio, particularly well versed in some groups of insects, he suggested a possible explanation for this phenomenon, assuming a sort of warming up before flying away in displacement and courtship.
Formulated this hypothesis, in the next days I devoted myself to a more accurate observation. Actually I was able to notice that, after having made some short flights, some subjects landed on the leaves exposed to the sun, almost imperceptibly vibrating the wings but at a considerable “speed”, at the same time emitting a particularly high hum in frequency with respect to the frequency in Hertz at which we are used to hearing the normal buzz of flight. This hum then tended to increase more and more in frequency until it reached a point where the subject suddenly took off, instantly changing the buzzing to the classic flight buzzing.
So, may be a behavior to be defined as described as a sort of warm up, or may it have to do with a territorial / reproductive behavior? In the absence of any bibliographical information, I will continue my observations, if only to satisfy my curiosity!