Rough idile when fan is on

Hello, the fan bering on would increase the load on the engine, which would expose any engine performance concern when the vehicle is under load. Testing is required to find out what is going on.

Next time turn fan on before starting engine - is fan blower out of balance due to foreign debris - which finally gets re-positioned and smooths out? This imbalance can vibrate to the point it can be felt inside the cab - should be accompanied by roaring noise .. Vehicle “sat for around a year” plenty of time for mice to make a nest in HVAC. Online guess..