Thanks that was a good check and a superb observation regarding te inducer fan,,, if it spun at less than 1750 rpm or so, as in 'slowly' the fan motor is bad, or for some very unusual reason not getting the full 110 volts it needs,
if the fan isnt working right, its Hall effect switch will not close allowing the gas valve to open,, if it doesnt have a hall effect switch, then the furnace has a pressure switch that will hold the gas valve off....
Having said that,,, it still doesnt explain the clicking,
so we several things going on at once here,, that dont add up,
Do this as a test, find the 24v transformer, run wire from both of its terminals to each of the two terminals on the gas valve,,, as a test only,,, for no more than 4 seconds, the gas valve should open with no clicking
If the gas valve clicks then it is a gas valve problem If the gas valve opens cleanly then its the hall effect switch on the inducer fan or the combustion chamber pressure switch causing the gas valve to repeatedly click.
do that check, get back to me