Montero A/C grumbles

I have a 2001 Mitsubishi Montero sport. Runs fine and dandy, but I have a concern. If I don’t use the car for a while, and turn on the A/C, I get a big old grumble the second I turn the knob. The A/C works fine though…any thoughts as to what may be happening?

also, it’s just one big noise in the beginning and that’s it…not a constant noise.