DIAGNOSIS The noise is almost certainly coming from the cooling fan. Debris has gotten into the fan blades or the fan motor bearings are failing.
FIX Unplug the unit. Locate the rear ventilation grille. Use a vacuum cleaner with a brush attachment to thoroughly suck out any dust, pet hair, or crumbs from the exterior vents.
IF THAT DIDN'T WORK If the noise persists, the internal fan may be misaligned or have a damaged blade. This requires removing the outer casing to inspect—a job for a technician, as you'll be near high-voltage components.
HEADS UP Don't ignore a grinding fan. Overheating from inadequate cooling can damage the magnetron and other expensive parts.