I'm not an A/C expert, but are you hearing the compressor clutch engage? The pulley will freewheel until the clutch engages, then it will charge the compressor.
I'm no expert either. This is literally the oldest car I have ever had usuall all my cars are newer and come stock with ac. I'm not a great mgiever so there is definitely a lot that I read here which goes in one eyeball and out the other haha .