What would happen if I add essential oils to this humidifier?

Essential oils will corrode the humidifier, causing the water tank or base to crack.

Could I put the humidifier on the floor or a wooden table?

Yes, but it isn’t the smartest move: 1. Best place to put a humidifier is where the most airflow is. 2. Potential of wetting the surroundings due to locally high humidity levels, it is recommended to put a tray underneath it.

Is the bubbling/knocking sound it makes while running normally?

It is normal for the humidifier to have bubbling sounds sometimes. It is the sound of water moving from the water tank to the base. Other times it is quiet and does not affect its performance.

Can I use tap water or is distilled water preferred?

It depends on where you live. In some areas, water is richer in minerals. The humidifier won’t instantly go haywire if you do use tap water, but in the long term will cause scale and you need to maintain it more often. Distilled water is recommended.

Is it normal to have standing water in the base when you refill the water reservoir?

Yes, it's normal. The water flowing from the tank to the base when you add water and place it back is part of the humidifier's operation. It draws water from the tank and disperses it through the base to increase room humidity.