Hey guys, I have an 07 F150 4.6. last couple days i noticed a fluttering kind of noise coming from under the hood. So i took a look and i noticed that the face of the AC pulley was spinning, and the stopping over and over again. the belt was continuously moving, but it was just the face of the pulley that was on and off spinning. I don't know if that is supposed to be spinning or not. Any help would be appreciated, thanks.