The main mistake was not being armed to begin with and delaying going for the rifle until the tapping repeated itself.
As far as the doorknob turning, that's scary. Crooks are often cowards and do little steps to build up their courage for something big. That's why someone driving by you several times is a bad sign, or a doorknob turning without the door opening. Next time it might open.
I do agree that lots of ovens make a tapping sound when heating up. I guess it's the metal expanding.