The sound is steam forcing its way out
A drop of water on a hot pan does not simply vanish. The part touching the hot surface turns to vapour almost at once, and that vapour has to go somewhere. It escapes through the water above it and through whatever gaps it can find, and the turbulent rush of gas is the hiss you hear. This is described directly in the context of hot road surfaces, where water trapped under a hot prime coat or asphalt overlay produces "a hissing sound as the water vapour escapes" [1].
The everyday version is the same event at smaller scale: a droplet on a hot stove, a splash on a hot iron, or water spilled on sun-baked pavement. The hiss is not the water boiling in a gentle, bubbling way. It is gas moving quickly through a narrow, messy path, which is why it sounds like a sustained shhh rather than a series of pops.
A cushion of vapour can make the sound sharper
When the surface is much hotter than the boiling point, the water may not touch it at all. A thin layer of vapour forms underneath and holds the liquid up, so the water is effectively floating on its own steam. This vapour film is unstable, and experiments with water on a very hot metal hemisphere show that the film can oscillate violently and then break down [2].
That breakdown is a more abrupt event than steady escape. The abstract of that work reports acoustic effects including "clicks" and "hissing" during these vapour-film changes [2]. So on a very hot surface you may hear two things blended: the continuous hiss of steam escaping, and sharper noises when the vapour cushion collapses and the water briefly slaps against hot metal before a new film forms.
Why the hiss changes with temperature and surface
The character of the sound depends on how fast vapour is produced and how easily it can get out. A hotter surface drives more vigorous vaporisation, and a surface that traps water, such as a porous or covered road layer, gives the vapour a longer, more constricted path out, which is the situation described for blistering pavement [1]. In that setting the escaping vapour is forceful enough to blister and break up the surface itself, not just make noise [1].
The vapour-film experiments add that the outcome also depends on how far the water is below its boiling point and on the geometry of the hot object [2]. That is why the same splash can hiss steadily on one hot surface and crackle or sputter on another. The underlying event is the same, but the balance between smooth vapour escape and sudden film collapse shifts.
Sources used in this answer
Water vapour blistering of road pavement prime coats and bitumonous surfacings
Water vapour escaping from hot prime coats or asphalt overlays produces a hissing sound and can blister and disintegrate the pavement surface.
Spontaneous triggering of vapor explosion: Results of experimental studies
In experiments on water interacting with a hot metal hemisphere, the vapour film around the hot object can oscillate violently and break down, producing acoustic effects such as clicks and hissing.