Most williams wall furnaces are natural draft and have no fan motor. I will assume that yours has not motor. The high pitched noise could be gas pressure related. The next time you hear the noise, simply open the grill that allows access to the burner compartment and try turning the gas cok ( the gas shut off valve ) just a little towards the closed position. Do not go all the way to the closed position as you would have to relite the pilot. If the noise goes away you may need a pressure adjustment.