

Electrical humming noise from furnace
I have a ruud high efficiency furnace that's 6 years old. Yesterday I noticed a faint electrical hum coming from it, even though the heat is off and the fan wasn't running at the time. I've never noticed this sound before.
The fan is set to run 50% of the time, and the filter was changed a few days ago (every 6 weeks). The digital display on the board shows 0.
The sound seems to be coming from the transformer at the top of the board pic. I can't be 100% though
Is this normal and I just never noticed before, or should I have it check out? What might cause the hum?