I agree that it sounds like the decoupler/clutch on the alternator.
Re: serpentine pulleys, it may be worth pulling the serp belt in order to spin the idlers (etc). Listen to each idler as it spins (for any catching or 'gritty' noise). They begin life packed with grease and don't spin freely for long. Many miles later, most of the lube has migrated out past the seals and it enters a stage of life where it's almost dry but Ok mechanically. In this stage it spins very freely and will continue spinning for some time if allowed to do so - where previously, the grease and seal would stop it in seconds. I'd replace it if they appear dry and spin freely enough to continue spinning for 5 seconds or more - because I think that from that stage to failure (gritty, catching operation as the bearings fail) isn't as far away as you'd like. The pulleys are relatively cheap at NAPA, and I've seen more than one failed serp belt take out the timing belt/rockers due to the jolt when it failed and wrapped around the crank pulley/dampener. Insurance!
You can spin the rest too - viscous heater, A/C pump, P/S pump, etc; they're a bit more challenging when trying to detect subtle signs but if they're failing you'll see/hear/feel something amiss while turning, perhaps even a wobble off-axis if it's really bad.
You can replace the decoupler in 1/2 hour if you have the tool, or the whole alternator (as some do) - which comes with the decoupler. Careful, as some aftermarket alternators and pulleys don't last.
You can do the diagnostics on the serp belt-driven idlers and accessories in another 1/2 hour or so, and change most any idler in 15 minutes once the belt is off.
Good luck,
Mark