Okay so I am new to the Mustang world and just have a quick question.

When I turn on the AC the Pulley stops spinning for a couple seconds then goes back rotating for about 10 sec. It does this on and off and will make a hiss sounds when it stops spinning. Is this bad for the belt or is it supposed to do this?

 

the off and on cycling is normal, if you put the A/C on high, and it still keeps rapidly cycling, you may be low on freon, the hissing sound might be (with out hearing it), when the clutch releases the plates slip a little and it might be making that metal slip sound, or the bearing might be going bad.