When the blower motor is on (I use it on the lowest setting in auto mode) there is a noticeable vibration on the steering wheel when the car is idling. What's interesting, I only feel it when the engine is on and running idle - when start-stop shuts the engine off and the heater keeps running, there is no vibration.

Otherwise the idle is super smooth, when the heater is off, I can't even tell if the engine is running or not without looking at the dashboard  (I had the valves cleaned via walnut shell blasting a few months ago).

 

I would not think the blower draws so much current that could influence idle (in the lowest setting, how much could that be - 1 amp?) - but I cannot really think of any other reason. The cabin air filter has just been replaced about a month ago.

Could be aircon compressor, it's driven by the engine, so while the heater blower will run with engine stopped the aircon doesn't.