If you hear a buzzing noise immediately following engine shutdown, the bearings have failed - If the pulley will not spin freely at, the internal spring has locked up. (causing increased tensioner movement or belt span vibration) - If the pulley won't spin freely in one direction and engages the alternator in the other direction - All are signs that the pulley requires replacement.
IDP or "Isolating Decoupler" Pulleys are also abbreviated as OAD pulleys; "Overrunning Alternator Decoupler" and can "decouple" or smooth the belt torque changes in either direction. A lower cost clutch pulley cannot offer this same benefit; it "slips" in one direction only. Only a true IDP/OAD pulley can provide the belt system with controlled drive torque providing higher efficiency along with reduced noise, vibration, and belt "slap".