Printer and UPS (Uninterruptible power supply) are not friends

I have an APC UPS. Don't know the exact model, but I've got it a few years back. It's coected to one electrical outlet. It is not networked in any way.

Then I have a Samsung ML-2165W printer, which is coected to a different electrical outlet, on the opposite wall.

Every time I tu the printer on, UPS makes an odd clicking noise. Like a capacitor going on/off. Click 2 second pause clack.

I'm pretty sure the outlets are on the same circuit, but I have not tested that yet.

The circuit is not overloaded. The only things plugged in are: laptop, router, cable modem. Unless, of course, the circuit is bigger than I think and joins some other rooms as well.

Nothing else goes off or makes any odd noise in the the apartment. The building is a newer one, built in 2008 (Canada). I'm pretty sure they would follow all electrical standards.

Any ideas why this could be happening?