Thanks for the advice so far. Rice guy, Im not an expert at detecting smells but the best comparison I can make is that it smells similar to a bottle that has been opened for a few days and is starting to oxidize. The taste is something similar to that as well.
Interestingly, the carboy in question has an old rubber stopper. I have 2 other 1 gallon carboys with silicon stoppers. All have the same airlock. The wine in the 1 gallon carboys with silicon stoppers seems to taste the same as it did a month ago, fruity and pleasant. The 5 gallon carboys with the rubber stopper is the one in question. Is it possible the rubber stopper is past it's useful life? I already replaced it with a silicon stopper just in case.
The wine in the gallon carboys does not have oak, but the 5 gallon carboy is question has an oak stick, so maybe like gilmango said it could be the oak.
Lots of variables to consider...