Yep, that's a damned good clue that you may need a recharge... Normally when correctly charged, liquid refrigerant is forced through the expansion valve and is silent but when you are down to the last gasp of refrigerant then refrigerant gas or vapour is passing through the valve giving the hissing that you can hear (Behind the glove box, right?).... Your aircon compressor is lubricated by lubricant carried by the liquid refrigerant, no liquid equals no lubrication equals wrecked compressor equals big repair bill. Get it sorted.
Generally speaking if it is cheap, then it will cost you, find someone locally who knows his stuff.