What are possible causes for belt tensioner "chatter" at idle?

Question

Moving from one set of issues to another on my LS1 Chevy.

With the gear in 'D', A/C on and the handbrake pulled, I popped the hood and noticed for the first time that the main belt tensioner is "chattering" (video link below):

LS1 Belt Tensioner Chatter

The car has had an issue with vibration for some time now. The symptoms are as follows:

severe vibration whenever the car comes to a stop (it seems to be related to the suddenness of drop in RPM)
at idle with the gear engaged in 'D' (~600 RPM). With the car in 'P' or 'N', engine speed is ~750 RPM and the vibration is lesser but still faintly discernible.
the vibration is felt throughout the car body and is amplified with use of the A/C.
I don't feel any vibration the steering wheel.

I was expecting that this was due to the failure of the vibration damper on my old crank pulley, but the issue persists even after the installation of a new crank pulley.

The vibration is felt with the car stationary in 'D', regardless of whether the A/C is turned on or off (the only thing that changes is the severity), so it would appear to be an imbalance of some sort.

Based on this forum post and the fact that the issue was present before the new crank pulley installation, I would say that crank pulley alignment is not to blame.

What could cause this behavior?

I've also put together an action plan. Comments and suggestions are welcome:

Inspect spark plugs
Inspect throttle plate, PCV, vacuum line
Replace belts, assess possibility of pulley misalignment
Inspect engine mounts
Inspect transmission mounts

Pᴀᴜʟsᴛᴇʀ2 · Answer

Does it make the vibration when in park? If not, I'd suggest you may have a vacuum leak after the throttle body or possibly your Idle Air Control (IAC) valve is not functioning correctly. If you are not experiencing any rough running at off idle, it has to be something which would only affect it at idle, or it could be slight enough that you are not feeling it above idle. If it is running slightly off above idle, it could be spark plugs/wires/coils (of which it would only be one of the eight of each and then just not functioning at 100%) or an injector might be partially clogged.

Answered by Pᴀᴜʟsᴛᴇʀ2 on May 27, 2021

Allan Osborne · Answer

The chatter on your serpentine belt tension is normal operation. As the engine speeds up and slows, imperceptable to the naked eye, during normal operation the effect from the crankshaft is absorbed by the tensioner and avoids a code P0300-Random engine misfire. You need to check your vehicles electronics with a scan, before deciding on a plan of action.

Answered by Allan Osborne on May 27, 2021

Brad · Answer

I drive a genesis coupe for some time now I have been experiencing the same sort of issue- though through forums I have somewhat come across it being normal.

I have had my spark plugs replace have had a major emissions clean done on my vehicle as well as other maintenance things.
Engine wise its bone stock-exhaust included

I changed out an idler pulley due to having some nasty loud bearing. 
After replacement I noticed no change in shudder- even so much as my idler pulley again is noisey loud. 
Upon further inspection of my drive belt- I may have come across the pulley being misaligned. So I fiddled around and now believe it is aligned.

Now your probably asking well - did it solve the shuddering? In part yes- as I come up to a complete stop and or driving at very low speed I do not get a shudder-as I turn my fan on/ defrost for climate control,i still get a shudder although seemingly less severe. When I place my car in reverse or drive shudder seems to almost be non-existent.

So I'd say check your replacement pulley/others for alignment it's not much to go on but it may help.

What are possible causes for belt tensioner "chatter" at idle?

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