Serpentine belt not turning pulley all the time?

My 2011 truck w/ ~35k miles on it has a belt chirp/squeek noise. It's been happening for about 6 months, and only does it for ~10 mins on wet/damp/rainy days.

I haven't replaced the belt YET, but upon inspection of the serpentine belt while my truck was running, I noticed that one of the pulley's was not rotating all the time while the serpentine belt was moving.

It's the lower right pulley, so I think that's the A/C compressor. Is this normal? The squeek gets louder when that pulley starts to rotate.

 

whatever the belt touches must rotate or the belt would burn up


the AC is a pulley with clutch, and underneath the rotating pulley is the
AC compressor clutch and input shaft, that only engages when the AC wants to pump


pulley must always rotate but what it is connected underneath only rotates when needed for AC

 

Ok so I must have just not saw the pully moving and only saw the circular metal piece moving when the clutch was engaged.

 

UPDATE: I replaced my serpentine belt @ 40k miles and no more squeak!