This may not be overly serious, but it's definitely not normal either.
The vibration/rattle always happens when the RPMs momentarily drop below ~650 (when engine is under added load due to turning wheel or switching on A/C at idle). There might be a momentary "shudder," but the engine quickly stabilizes like a properly-running car ... then the vibration/rattle will eventually stop ... then start again ... then stop. Intervals of around 30 seconds or so.
It certainly might be a "mystery noise" that doesn't affect the long-term health of the car, but I still get an occasional funny look when I stop at a gate/toll booth/stoplight/etc and the vibration/rattle starts. Annoying.