Strange sound when A/C compressor is on
 

Hey guys,
Please help me, I'm the proud owner of a Mazda 3 2009 Hatch 2.0L and for about some time there has been a sound coming out of the compressor when the a/c is on, but when is off the sound goes away. My mechanic told me that it is the compressor and that I definitely have to change it, but the a/c is really working well day by day, the only problem is the sound, so I'm not pretty sure that's the issue, so any of you maybe know the answer to my predicament?.

Here is a video I made, you can see that when the ac is off the sound stops. I'm new in the forum so if the video is not visible I apologize.:bat:

Mine squeaks when it's on but that's it