When I turn the heat on, my furnace just hums. It would take a few seconds to "warm up" in previous years before the blower actually started blowing but I have let it hum for a minute or more & it doesn't come on. The a/c works fine, now and this past summer, but I did have to turn it off manually b/c it wouldn't shut off by itself. Any ideas what it might be? I have an old dial t-stat, not programmable one, just in case that's important.