Hissing doesn't have to mean imitating an animal
A hissing tic could simply be to make a ‘hissing’ sound, similar to other vocal tics such as whistling or humming.
In fact, a simple tic would be to make the noise. A more complex tic would be to add movements or specific behaviours to the hissing tic.
Vocal tics are very common, and very easy to carry out. Sometimes, individuals will mask such tics, or try to supress them in public.
Tics vary, but come from the same processes
Although this tic may seem unique or ‘rare’, it will no doubt be shared by a large number of people with tic disorders.
It is also true that tics can change over time. In frequency, in severity, and in nature.