When you turn on the AC the compressor engages and it will make a slight noise, that's normal. It's also normal for the compressor to occasionally cycle off for a few seconds and back on again.
As long as your AC is working properly and maintaining the right temperature there is probably nothing wrong with it.
OTOH, if you have had the car for a long time and it has only recently started making this noise, then something may indeed need attention. Possibilities include a worn or loose AC belt, defective compressor clutch, and too much or too little charge in the system.
So, if this noise is something new and it has you concerned, have a shop look at it.