We have had this heating unit for 28 years and have only changed the blower on it. Then all of the sudden it started making loud banging noise when it's on the cool down. It's so loud it wakes everyone up at night

That model number looks like a Coleman mobile home furnace number. At 28 years old, if it is the furnace I think it is, has out lived it's normal life expectancy by at least 50%. Without being able to hear the noise, it's hard to even guess what it might be. The fact you have replaced the blower at some point would lead me to look at it first.