Installed in 2004. Just before the fan comes on the furnace bangs/clunks - sound like someone hitting pipes with a wrench. We have had trouble with ignition. Could this be related? If not, any answers?

Could be simply a duct expanding or contracting causing the pop noise. I have very few ignition problems with that year Rheem. It is also posible the blower wheel is not adjusted right or loose. Furnace has a 5 year part warranty. Go to the furnace and put your hands on the ducts as the blower turns on to see if that isn't the cause.