Probably you'd need to take it down, if you can't see anything else obvious.
Be aware of the electrical hazards of mucking about in microwave ovens. Even with the unit unplugged, there's the potential for a lethal charge to be stored in the capacitor for a long time. Usually they have a bleeder resistor to discharge the capacitor when it's not in use, but if it failed the oven generally would still work fine and you'd be none the wiser. Microwave ovens are said to be about the most electrically dangerous common home appliance to work on...which is not to say it cannot be done safely by ordinary people, just that one needs to take care and be wise.