DIAGNOSIS You're hearing the convection fan hitting something. It's almost always a piece of food debris—like a small bone fragment or a piece of hard crust—that got sucked up into the fan housing at the back of the oven.
FIX First, unplug the oven and let it cool completely. You'll need to get to the back panel. 1) Remove the crumb tray. 2) Use a Phillips head screwdriver to remove the four screws on the back panel. 3) Carefully lift the panel off. You'll see the fan assembly. 4) Shine a light and look for the offending object stuck in the fan blades or resting in the housing below. 5) Use a pair of tweezers or needle-nose pliers to gently remove it. Don't bend the fan blades. Reattach the panel, screws, and crumb tray.
IF THAT DIDN'T WORK If the noise persists and you found no debris, the fan motor bearing might be failing. That's a less common but known issue. You'd need to contact Breville support for a motor replacement; it's not a user-serviceable part.
HEADS UP When you're air frying small, light items like herbs or breadcrumb coatings, use the provided mesh tray or a piece of parchment paper to keep them from flying around and getting pulled into the fan intake.