Toilets make a plethora of strange sounds that are mostly related to the movement of water and air through small pipes. In an apartment in NYC, you can hear noises that range from whooshing to gurgling, but if you bear witness to a hissing, it typically means that air or water is leaking from the fill valve.
The only way to verify that your fill valve is malfunctioning is to remove the lid from your toilet and inspect the tank. You must pinpoint the source of the hissing in order to begin to fix the problem. If you notice that the tank is too full, it means that the fill valve is leaking and needs to be replaced. In that case, it is encouraged to contact a NYC plumber to assist with the repair.
When the plumber comes to fix the toilet, he or she will adjust the float, in order to stop the fill valve from over flowing. If you have an older, ball float design, the armature and ball must be adjusted to stop the water from running. If the problem is air in the pipes, the plumber will have to do more extensive repairs and diagnosis in order to alleviate the annoying hissing noise.