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I have been experiencing the same thing and here is what I found. First, there are several small exhaust leaks around where the Y-joint enters the canisters. I have since had these welded. However, during my last visit to the garage, the mechanic stuck a tiny camera inside the exhaust and said that there is a whole bunch of loose metal material in there that he had to vacuum out. I suspect that something inside the exhaust is coming apart from all the heat.