DIAGNOSIS That roaring or rattling noise usually means the centrifugal fan wheel (the squirrel cage blower) has come loose on its motor shaft or something is obstructing it. Grease and debris buildup is the prime suspect.
FIX First, remove the grease filter and wash it in hot, soapy water—clogged filters force the fan to work harder. For the internal fan, you'll need to access it. Unplug the unit, remove the two screws holding the vent grille (underneath, toward the front), and carefully lower the grille. You'll see the fan assembly. Check for any obvious debris like a piece of foil or packaging. Use a vacuum crevice tool to gently clean around the fan blades. Tighten the central hub screw if it's accessible and seems loose.
IF THAT DIDN'T WORK If cleaning didn't help, the noise might be from worn motor bearings. At that point, the entire blower motor assembly would need to be replaced, which is a more involved repair.
HEADS UP Be super careful when lowering the vent grille—there are often wiring harnesses for the lights attached. Support it and disconnect any plugs before fully removing it to avoid stressing the wires.