Bill,
When you turn on fan only do you hear a "hum"? That's what I heard. I've spent some time on the Oregon Coast this past summer/fall/winter with its salty air and after having searched the problem online, and having both units simply HUM at me without the fan coming on, went to the roof, took off the shrouds and manually turned the fan shafts which were frozen up and rusty looking. The shafts were so frozen up due to the rust that I thought they may have been gear driven and I would break something if I turned them, but NO, its an electromagnet fan and both were very stuck! They should turn easily.
Cleaned them up and no more issue. Others have replaced capacitors used to "start these up" but if you can and especially if you HEAT A HUM when you turn on the fan only, try this quick method first.
Mike