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If we are talking about the same thing, its a very loud, scary sound, at least it was for me with my head under the hood. Anyway, if that is whats happening it is indeed a vent to relieve unwanted pressure from the system. In my case I had replaced the oriface tube filter and evidently bent it puting the line back together, which didn't allow to system to flow properly and thus the venting. You may just need to have some routine service, i.e. new oriface filter, pull a vacuum and recharge the system. worst case if the compressor let go and clogged the filter with fallout you may have to replace the compressor