Sometimes At Idle In Gear With A/c On Get Bad Vibration

This happens when stopped at a light, can feel it in steering column and hear it from cracked windshield defroster vent. In the old days we would just bump up the idle speed 50. RPM or so. With throttle by wire I am at a loss, almost forgot, v6 with autobox.
Thanks for your time
Richard.
 
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