Clicking sound while A/C is on...

[sdamos]

Greetings all! - Ford Explorer Sport Trac - 2005

I live in Arizona, use the A/C a lot. Just got a new A/C compressor and clutch. Not sure if this was happening before, but I started listening closely since the unit is brand new. The pully on the power steering motor, which is opposite the engine of the compressor, has an additional disc on the front of the pully. Every 20 seconds or so, with the A/C on, that disc clicks on and spins for about 5 to 10 seconds then stops. Then, about 20 or 30 seconds later it clicks and does it again.

There is not much of a draw on the engine, but while going slowly down the road you can hear it if you listen closely. What is the small disk on the front of the pully on the power steering motor and what does it have to do with the A/C because it does not do that with the A/C off? Please advise...

Thanks,
Shannon