My hvac makes a hissing noise when turned on what could be the cause?

A failed or failing or sticking valve inside the compressor motor.

Abnormally high pressures on the outlet side of the compressor motor activate an internal pressure-relief safety valve.

Abnormal and very-high pressures in the compressor motor, possibly heard as a combined hiss and shriek.

A refrigerant leak.

Hissing at the thermostatic expansion valve, typically low in volume and short in duration, and traced to the TEV or cap tube.

Hissing sounds from ductwork: look for an air leak in both supply and return registers. Even a small air leak can cause loud noises. This sound will occur when the fan in the the indoor air handler unit is operating, and it may change depending on the position of various duct system registers and dampers, blower condition, even air filter condition.

I would get it serviced.

I would definitely call the folks that replaced my thermostat.

Refrigerant leaks: Escaping gas might be the actual problem. Leaks along copper refrigerant lines will allow gaseous refrigerant under high pressure to escape—and this is a major problem. When an air conditioner loses its refrigerant charge, it can lead to damage to the compressor that will put the entire unit in jeopardy.

Internal valve leak: The compressor valve can start to leak, causing a hissing sound. This valve controls the pressurization of the refrigerant, so this needs to be repairs ASAP.

Compressor problems: It could be that the compressor itself is malfunctioning because the pressure inside it is too high. The compressor is the most expensive part of an AC to repair, and usually it’s more cost-effective to simply replace the air conditioning system entirely. If the hissing sound you hear turns into a shrieking noise, shut down the AC and get our repair team on the phone right away.

If you’re only hearing periodic hissing sounds that last for several seconds, it’s likely just the expansion valve, which isn’t a problem. If the sound is sustained, however, you need to have HVAC technicians on the job to find out what’s wrong and how to fix it.

The t-stat has nothing to do w/your hissing sound, it depends where you hear that sound, if it is outside you could have a Freon leak. If it was installed by a LICENSE H VAC Contractor call them & ask for a professional option.