Humming when ac kicks on engine fan

Sounds strange. Does it make that noise when the fan engages for engine temperature?

 

Hose to fan hub leaking ?

 

If the A/C is kicking the fan on, and that's the only time you hear the noise, could it be your expansion valve?

 

And if thats the case, the conclusion would be its on a slightly low charge because the compressor never gets all the gas compressed to liquid ONLY WHEN FAN IS ON and the hum is gas burbling through as well as liquid.

(Sorry gpd, just wanted to finish that line of thinking so people know WHY that would happen)

 

No, that's good. I didn't know why they made noise, I just remember they did. It's good to have the other half of the story.

 

The fan normally kicks on when the refrigerant pressure reaches 250 PSI.
The pump should shut off at 300 PSI to protect the system. I am wondering
if the switch is failing and turning the fan on when the pressure is too high

 

Technically its possible the valve is stuck too open, never really allowing liquid to build, but since its intermittent with fan, less likely.

Gas to liquid and back is weird, especially when we start tampering with it in applied engineering.