My bathroom fan that came with the house (house built in 1965) doesn't start for about 5 minutes after I turn it on. When flipping on the wall switch, you can hear a very low hum like something is running. Then about 5 minutes later, then you hear the louder sound like the steam is now being drawn out of the room. From that point it works fine. I replaced it with another vent, but it works in the same manner. What could be wrong that would delay the fan from working properly? I've oiled it. I've climbed up in the attic and am getting the correct air flow up there. Any ideas?