Hmm...it's very weird that your sound goes away with the A/C. Is it a gurgling sound? Remove the belt going to the power steering pump, then start the engine to see if the sound goes away. If it does go away, then it's your power steering pump either failing or not getting enough P/S oil. It's also possible that water has somehow managed to enter into the power steering system and has contaminated the oil.
If the sound persists after removing the P/S belt, I'd suspect something is up with the clutch on the A/C compressor. (Maybe water got into it?)
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