There's a serpentine belt that drives various bits and pieces including the air conditioner. When this belt gets old it becomes slacker, and the sound you hear is the belt slipping. It's a very cheap and quick job to get done, or look on youtube for videos if you want to do it yourself.

I can't be sure about your model, but a lot of cars have screws that allow you to adjust the tension of the belt. It could be a simple matter of of your brother loosening those screws, tightening up the belt, and being done with it. The same thing happened to my old Starlet ... my dad fixed it in all of ten minutes, so it's *that* minor a job!

Sounds like the fan belt. Could be worn and is rubbing. Very easy fix

sounds like a squeaky A/C clutch. or possibly a tensioner pulley going bad. Easiest to repair is the tensioner assy. If it is the clutch bearing going out, it gets more complicated and expensive to fix.

Is the belt slipping?

I have a 2002 Toyota Highlander that I recently bought from a dealership. Nothing seemed wrong when I bought it, but it's been a few weeks and recently I noticed this noise that only happened when I turned the ac on. It stops immediately when I turn the ac off. It's sort of a squeaky click that is repetitive. Does anyone know what could be causing this? I'd rather not have to take it to a mechanic, my brother works on older modeled cars. Thanks! :)