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the hissing is most likely a leak of the AC system.
when the freon is low, the compressor will not engage.
most likely, the freon is gone.
you should have an AC shop figure out where the leak it and fix that. could be a hose or a component of the AC system. once the leak is fixed, then charge it with R134. R134 will be cheaper and easier to find if you need to recharge in the future.
my ac shop charged $45 for diagnosis. they used a sniffer that pin point the leak.
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