Why is a bird (titmouse) tapping on my grille

most likely it is attracted to a shiny part of the grill, not so much the grill itself. My parents are currently trying to convince a pair of cardinals that the shiny chrome luggage racks on their vehicle are not there for the birds' amusement. If the bird is making a mess in the area around the grill (like the cardinals on my parents' car), try hanging a mirror nearby, but far enough away that the mess that birds inevitably leave behind won't be in an inconvenient place. The reflections and shininess of the the mirror will capture their attention (hopefully) and get the bird(s) to leave your grill, and develop a new obsession.