I have a vented gas log for 2 years with no problem. But this year, it start making roaring sound after it been lighted for about 15-20 minutes. It sounds like the gas rushing and it lasts for 15-20 seconds or so; than it stops for a
10 or so minutes and starts up again for 15 20 second. I opened the vent/damper all the way but it still make the roaring sound. Other than the sound, everything seem to work. Any suggestions will be appricated.