I have a Heatilator NDV3933IL that was installed about 10 years ago. The other day it was lit and burning for about an hour and then it stated popping and the flame was going in and out. I turned the switch off but there was still a tiny flame lit. It almost looked liked a pilot light. It kept burning. I had to shut the gas valve off for it to go out. I let it go for a day until I had a chance to look at it again. I turned the gas valve back on and the flame came on for a couple seconds then it started popping and went out except for the tiny flame. I fireplace does not have a pilot light. Does anyone have any ideas? I appreciate any help!