Motor timeout

Hey all,
My Smoothieboard has pretty loud stepper whine even when the steppers are not actively moving (though it’s probably more the steppers being old and cheap). I was hoping there was a feature in smoothie to release/disable the motors (like what M18 does) after a period of inactivity.

Is anyone able to confirm/point me to the docs for a feature like that?
I’ve done quite a few google searches, as well as looked through the docs pretty extensively, so maybe the nomenclature I’m using isn’t quite right.