Yeah, this one's common. It's not really 'off,' it's in standby. This model uses 0.8 watts in standby mode so it can wake up quickly for a print job or listen for an Alexa ink alert. That's the hum you hear.
If you want zero power draw, you have to unplug it from the wall. There's no physical power switch to cut all current.
For context, it uses 7 watts during operation, which is higher than a basic, non-wireless printer. It's a trade-off for the auto document feeder, fax, and Wi-Fi features. If the constant hum bothers you, plugging it into a power strip you can switch off is your best bet.