hum from A/C compressor ?

[AnobleL74]

Hi
my 66 has a 350 with March pulley system and Sanden compressor with Classic Auto Air. i have had the system evacuated and recharged to mfg spec recently.
it blows cold when low ambient temps, but tends to not blow cold enough when ambient temp it high upper 80's.

it has been making a grinding noise sometimes when i start it up, with AC on , and after it is already at operating temp. the noise is intermittent , not every time.

when i turn ignition off and AC off, i can hear a electrical hum coming from the compressor housing , i have to use a stethoscope to hear it , but definite electrical hum in there?

any thoughts, ideas or suggestions are greatly appreciated.
thanks
Allan