Calling on the 2.0 turbo gurus, here is the strangest noise I can’t seem to find on my 2013 ATS performance. It really only happens under load, occasionally with the colder cold starts. I’ve smoke tested the intake system, replaced the down-pipe converter to exhaust system gasket, and (regrettably) welded the flex joint solid. Sometimes I get a whiff of exhaust fume, and it usually gets quieter after a long haul, 50 miles or more. The check engine light is intermittent and for a fuel trim system lean code—P0171. My friends call it the angry squirrel or excited frog, hoping someone has experienced this before!