New Noise - Amana Refigerator

My elderly Amana bottom freezer fridge has started making a weird noise once or twice a day. It sounds as though a fan is starting up and having a hard time. It’s as though the blades are hitting something. It gets louder as the fan reves up to speed and lasts for about ten or twenty seconds. Then it either gets quite or shuts off - I’m not sure which. Other than that the fridge is working properly, with proper temperature maintained in both the fridge and freezer. Does anyone know what’s going on?