When I turn on the ac I hear a hissing sound can that be a sign that I am out of freon?

The belt driven compressor may not be producing suction, does the fan motor change speeds when you move the selector settings?

Whenever you add Freon,you are suppose to leak test first.Your compressor is good,you have a leak somewhere.A good place to take it to is Midas,they changed my freon over from the old stuff to the new R-134a,they leak test it first,that is rule 1 when adding freon.

It's possible that there is a leak somewhere in the AC system and when you turn the car on the high pressure in the lines pushes some of the freon out. Normally you don't need to add freon so my guess is you have a small leak somewhere.

You've got some of the picture, but not all of it.
You need to know what the high and low side readings are, so yes you do need the gauge set. If the system is charged, and the low side is not dropping below 25psi, I would guess the low side pressure switch is faulty. But if the low side is dropping below 20psi when the compressor stops, the system may be low on freon. Which means you have a leak somewhere.
The hissing noise is freon flowing thru the system which could also indicate a low charge.

needs regas aswith the hissing it has a leak

i beleive these are a vacuum system. if air temp is in cold position you will get no heat. listen for hissing noise with motor turned off but key on. if you hear hissing noise you have a hose broken or off. you will have to find it

The hissing is caused by the freon going through the orifice tube, so do not touch the 2 nuts. You will loose the freon in the system.

What's probably going on is the mode actuator is not functioning correctly.

Take it to a professional, a dealership should be able to help you.

you could have leaks at evaporator.

this is a sign that the a/c system might be low on freon. I would have it recharged. Asked the tech to add dye during the recharge that way if the system gets low on freon he'll be able to use ans specail black light to look for the freon leak.

you are going to have to put tracer dye in the system and check for leaks
if no leaks then might be low on freon and or there is a resistance in one of the lines