a/c making high pitched screeching sound intermittently

What is your question?

when i first turn a/c on, makes high pitch screeching sound that fades out; doesnt just stop, noise fades. i will turn a/c off during drive, then on again when it gets too hot..i hear this noise when i turn a/c on each time during same trip.a/c works pretty well. could be a bit cooler/colder; but good, not enough for me to take it in for more freon..is noise indication of something worse to come? should i take it in NOW? ive had the freon topped off once b4, yrs ago. (or so i was told...dealership, not sure they did anything). ty