AC possible issue?

On occasion after my runner has been sitting for several days, when I turn the AC on, there is a quick 1-2 second grinding in the engine bay, then it stops. It is only upon initial turning on the AC after the car has sit for several days. It won't do it anymore after that, even the next day it's fine. Only after sitting for 4-8 days does this happen. This is a brand new runner, 500 miles on it so far.
Any ideas?

 

If it were my 99 I'd wait for 8 days, then loosen the tensioner and sling the compressor by hand and see if that is the source of grinding...but since yours is a 500 miler...dealer it.

Or if the dealer route is a bitch, step 1

Good luck!