Alright I searched all over for an answer for u and i found an old guy (my dad) who used to do nothing but Trane equipment and he has seen this problem before. After turning off the furnace or at least the gas, you need to remove the flue pipe a few inches above the leak all the way down to the rubber coupling that connects it to the inducer motor. From there replace the whole length of pipe and elbow make sure it is all glued tightly. Reconnect to rubber coupling in unit and install a new rubber coupling in the other end of replaced section. This is the easiest way to replace the section plus the two rubber couplings in the line make any future repairs or parts replacements easier to access. Oh and you must absolutely make sure the flue pipe slopes back to the furnace so all condensation will drain properly to the unit. Sorry for such a long message but I hope it helps you