Hissing sound right after i turn the ignition on like air escaping from somewhere. it started right after I added freon to my ac. What could this be?

either you have a serious leak and gas is escaping when the gas is compressed or you have a snake in the vent .

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.

A hissing noise is an indication of a vacuum leak ( probably in the manifold ) as I think as the manifold heats up the noise is lost because the gasket is now sealing off

A mechanic would check the system with a set of pressure gauges. If the compressor is running, the low side would read around 30psi and the high side would be around 250psi. That is how you know if the system is full of freon or not.
You may have to get a professional diagnosis to proceed. If you replace the compressor, the accumulator needs to be replaced and a vac pump is used to pull all of the air out before the freon is added.

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.

your ac pump may have went bad

bad ac clutch

Hi, if you have full pressure (40-50 PSI on low pressure side with engine running A/C set to maximum and blowing) you have enough pressure, but there is air in the system which degrades the refrigerant leading to no cold air. To remedy this, you must completely discarge the system and refil with new R134a which has the compressor/system oil added to it.
To discharge the system, shut off motor, attach your filler hose to low pressure valve and allow system to drain. (squeeze handle if you have that type filler hose) Allow it to drain until no sound of escaping air is present. Be careful not to have the end of hose facing you or others, wear gloves and have some shop rags laid down, pointing nozzle toward rags, that stuff can damage paint and skin and may be cold when escaping. After system has completely drained, start the refil process with engine running, A/C on maximum, blower on full. After about 1/2 of the first can of R134a or 1 full can, your compressor should kick on. Continue refilling with 3-4 cans total (about 3 LBS total) until you have a reading of 45 or so PSI. After each can, allow refrigerent to cycle through the system about 3 minutes to obtain an accuret pressure reading. Pressure is relavent to outside temperature also, optimumly the ambient air temp. outside vehicle should be between 72 and 80 degrees F. for proper reading. That should get you blowing cold air again!
Please rate this answer! Thanks, Mike

Sounds like you might have a leak. If you charge the system and works for a time ,(hour, week, year) and then quits it sounds like perhaps the freon is leaking out somewhere. Freon does not ever get converted to air. Freon is in several states of existance while it is in your system. It is a high presure liquid, high presure gas, low presure liquid, and a low presure gas, but it is still always freon.
The valve your friend is talking about is called an expansion valve. What it does is to take the high presure "liquid" freon and allows it to pass into a low presure zone called an evaporator at a regulated rate. When you take freon at a liquid state high presure condition and expose it to a low presure environment it "evaporates" ( thus the name evaporator) and it gets very cold when it does this. This is where the cold air you feel coming out of the vents is made.
But the freon is in an enclosed system. By this, I mean it is never exposed to air, moisure (water), or anything else. I goes through all of these changes while inside this enclosed system.
Therefore, if the enclosed system is good then the amount of freon inside it will not change. If the amount of freon doesn't change then adding more will only make things worse.
I would suggest having the system checked with a hologen leak detector and/or system dye to find the leaks. Remember, if you have to add freon, then the freon that was there before must have escaped from somewhere.
Find a reputable shop to check the system for leaks. If it is leaking bad enough to run low in a hour it should be an easy to find leak.

sounds like you have a leak concern if they are adding freon

however, it also sounds like a restriction in the system and/or a defective TXV

do you have pressure gauges... this is the only way to diagnose an a/c system without just throwing parts at the concern