There is a smell coming from the humidifer, it is attached to the furnace. The odor is not constant, and smells like rotten eggs. Before the winter season, I cleaned the humidifier, and replaced the filter. What else should I do ?

Is the humidifier mounted on the return-air side of the system? Perhaps odors are being drawn through the drain. If your domestic water supply is a cistern, you may have high sulphur content in the water.