I have a 2004 Subaru Outback LL Bean H6 3.0 & the AC is not blowing out cold air

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I have a 2004 Subaru Outback LL Bean H6 3.0 & the AC is not blowing out cold air. When AC is turned on there is a slight hum under the hood, but no cold air. There is oil residue on the line connection stretching from the front of the engine to the back. What do I do - I have someone that may be able to work on the vehicle for me, but otherwise would greatly appreciate the verbiage for this fix so I can communicate it to the mechanic. Thank u.