[Coco] transformer hum in a coco when off?

[Jeremy Michea]

I was using a coco 2 last night, turned it off and it was still plugged in. A pen fell behind the dresser my coco was sitting on and as I reached for the pen my ear was right up against the top of the coco, right around where the PSU would be inside and heard a very faint hum, almost like 60 cycle hum, that kind of thing. I then tried another coco 2 and then a coco 3, same thing, very faint hum coming from the psu area. Also tried the charger for my laptop and put that against my ear - same thing. Tried the coco 2 in other outlets in the house - same thing - some are grounded, some aren't. Doesn't seem to matter. Is this normal when an electronic device is off and plugged in? Home inspection of the electrical was normal by the way.