My old furnace as well as my brand new Lennox 2 stage top of the line furnaces gas valve CLUNKS when the furnace comes on. Repair guys have been here 3 to 5 times and last time the clunking stopped, but only for 1 1/2 days. Why is this happening?? How can it be stopped? So annoying as the furnace comes on every 10 minutes. Is this dangerous?

Karen