Refrigerant flowing through the system. Focused on the expansion device at the start of the evaporator coil.
To make a humming or hissing sound, like an arrow or ball flying through the air; to fly or move swiftly with a sharp hissing or whistling sound., A hissing and humming sound.
the air is depleting from your lungs
A hissing sound coming from a chest wound can indicate that air is being sucked into the chest cavity through the wound. This is known as pneumothorax.
A ferret that make a hissing sound means they are annoyed or irritated
Geese make a hissing sound.
maybe clogged air filter or carburettar is sucking air and needs repairing
The Walnut Spinx Moth (Amorpha Juglandis) makes a hissing sound by forcing air out of it's spiracles.
it makes hissing vocalizations
That is the normal sound of the pressure equalizing in the system.
A cracked air cylinder could cause a hissing sound in a shock absorber during rebound. Another possible cause could be the seal.
Snakes are reputed to make a hissing sound.
Madagascan hissing cockroaches make a noise by forcing air out of a special hole.