I'm probably wrong here, but I know that on some years of the Monte Carlo and Impala there was a TSB for the ISS. TSB stated that in order to remove popping and vibration from the steering wheel, lubricating the ISS would correct the issue. My understanding is that they did this because many people were replacing the ISS and the problem persisted. I know I had the issue on my Monte, sure enough a little lubrication and the problem went away.
Perhaps this is related to that?
06 Monte Carlo LT
+3.9l V6 - 187k - Defy/Motorcraft 10w30 - EcoGard Synthetic - OLM
16.5 CX-5 Touring
+2.5l I4 - 45k - SynPower 0w20 - PureOne - 8.5k/1yr