GMP HEAT EXCHANGER
This popular 80% furnace has been sold since the early 90s. The first few years of production have been suffering from failed crimp rings on the heat exchager. The cells are held together by the rings punched as you can see. But in the circled case, the ring failed allowing the metal to separate as it heats up. First symptom is tripped rollout switch because the draft is reduced and the flames aren't pulled in. Goodman was offering some labor allowances so check with your supplier if you find a bad one.