Ring doorbell 2 Mechanical chime stays "on" for about 10 secs (before the dong)

I've just installed this, and can see that when you press the doorbell, the chime "energises" - the plunger drives over and I get a "ding" - but then stays rattling in the "on" position for about 10 seconds before coming off and giving the "dong".I've checked the setting is "mechanical", but then get stuck in the loop of testing it and then confirming it doesn't work correctly, which takes me back to mechanical or digital.