Furnace won't light just a buzz when it calls for heat

I have a Goodall LP gas furnace, I think its 92% efficient. Last night (May 31,12) we had 42 degrees. So I decide to warm the place up a few degrees, adjust thermostat up, and no heat. I went down to the furnace, it was buzzing or humming. I looked in the little round window and there was no fire. The air conditioner and blower motor work. So did the flame igniter fail? What is wrong with it? If it matters my LP tank is 45% full, 500 gallon tank. Thanks for the help.

Make sure that the thermostat is in the heat mode and not the ac mode, first comes to mind...

Gas valve will not open until sail switch detects exhaust air flow from exhaust fan. That switch is a sticking point on almost all brands ( new hot water heaters also). Sometimes the switch goes bad other times there could be something blocking its ability to move and of course it could be the exhaust fan motor. The first 2 are the most common problems in a no heat ( gas valve doesn't open) Buzzing sound is likely the ignitor there is a timer in the circuit that will turn it off and it will try to establish the exhaust flow sequence again. I think there is a cycle limit also where it will go into shut down mode for awhile before it tries again.