When I make ventilation on, I hear loud buzzing (and annoying) sound. It goes off only when the ventilation is off. What is this and how do I stop this sound?

What is your question?

It happened couple of times last autumn and again today. The sound does not depend on the blower speed, but it starts earlier when the speed is high. Only pushing the "off" button stops the sound; but I need some heat during winter. It is constant and rather loud buzz.

How long has your 2011 Toyota Sienna had this problem?

In November - I even asked my mechanic about the possible reason, but the sound just stopped.