Why would you only do the cap and not the plugs? It's a simple procedure. As far as your noise goes - i'd bet on a parking or a brake issue - if noise starts as soon as you roll - bearing is also a consideration but you have to go through brakes to get to bearings anyway. Could also be u-joint(s) - you're just going to have to dig into it. As mention earlier does it get louder more pronounced when cornering? Or when accelerating?