Humming when ac kicks on engine fan

Probably best to post a sound somehow. Also not 100 percent sure where your ac fan switch is, i find it odd that you say it makes the noise just a few seconds BEFORE the fan kicks on, unless we are looking at an ecm and relay controlled fan switch causing electrical weirdness, THAT almost makes sense, except for the sound

 

I would want to know what the A/C pressures are with gauges.

 

That very well may be helpful eventually, but why do we need to know gauge pressures exactly before we point to the problem? On my truck, the difference between the last 16 oz and the first 16 oz is quite drastic, but the intervening 35 oz does quite little to significantly alter the gauge readings, and whats more, those readings at 2lbs show "within spec" and appropriately kick on amd off fan/freeze switch activation (compressor off) but are not the 4.2 lbs the system was designed to take

If i am completely off base, educate me, but i do not see why we need gauge readings to diagnose what at present seems to be a slowly activating pressure switch as pressure builds to the fan cut-in. (Everything except the description of the noise, which could actually be an electronic hum signifying exactly what im pointing to)

 

Ambient temperature could have a lot to do with it, if electrical, well those are a different kinda demon that can be quite sporadic as well