2004 Nissan Sunny AC

When I turn on the AC and the car is in neutral I hear a belt like sound in the engine. When I press on the gas the sound goes away. The AC blows pretty much warm air while in neutral but gets cold when I press on the gas. I went to a garage they told me the compressor is bad and there is a freon leak. i live in Saudi Arabia and i don't trust most of the mechanics here. The car is a 2004 Nissan sunny(sentra).