Hi All: I have a Lennox G26Q3-75-1 furnace. Just before the inducton motor starts, there is a loud noise (thumb) that seems to come from inside the walls. At first I thought it was the house moving (getting cold out, figured the house will groan a bit), however this has been a constant thing right before the furnace kicks on. I've sat next to the furnace listening for the thump..no luck. When upstairs I can hear the thumb quite clearly. And again, only happens as the furnace is about to cycle. Any ideas? Thanks, Glenn