E320S4 Engine/Pulley Chatter

[rivermasternc]

I got home tonight to the wife saying, "I want you to listen to my car." Great!!

There is a chatter that appears to be coming from one of the pullies on the serpentine belt. It only chatters at low engine speed. Off and on with car idling in park or neutral. More prevalent with the car in gear, stopped, idling.

It seems to be coming from a what appears to be an idler pulley above and to the right of the crank pulley as I'm looking at the engine from the front. It is just below and to the left of the oil filter. The back (flat) side of the serpentine belt runs across this one.

Is this in fact an idler? Why would it be chattering? A tool handle on the torx type bolt sticking out of the middle of this pulley reveals a pretty good vibration in it too while chattering. Do we need to park it?