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Well first I would check for parasitic draw to isolate that concern.
Shut all doors,remove key,disconnect battery preferably with the ammeter already hooked up in line so when the circuit opens the current already flowing through the meter.( to avoid the initial rush of current through the meter and possibly blowing the fuse!)
(Ammeter goes inline between the disconnected battery cable and that battery post where it was connected to. If you're not sure how to do it,or have no multi meter then have a pro do it.)
Wait a few minutes so all the modules can fall asleep,then you want to see something less than 30 milliamps depending on how many gadgets the car has like onstar,remote keyless entry etc. ( on good GM cars I tipically see 7-16 mA or 0.007-0.016 Amp draw)
If that's good then based on your description of the sound maybe the A/C evaporator core has a slow but audible leak and the seller was "kind" enough to fill the system with freon before the sell? Hope not but there is a possibility.
(I'm not accusing anyone as I don't know where the car came from)
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