We have a 2 year old furnace that is attached to the basement ceiling. When it shuts off there is a very loud combustion type noise. It's scary it's so loud. Any ideas? Thank you

My best idea for you is to call for a On Site Inspection of what that furnace is doing. A loud bang is never a good sign. Now it is possible the blower cage is loose on the blower motor since that can make a sound when turning off. To check that turn off power and remove the blower motor access door. Then check to see if the cage is loose.