So I recently got my exhaust repaired, the previous owner had straight piped it. Now I hear a knocking or shaking sound from the engine bay very infrequently, only heard it once MAYBE twice on my 25 min drive this morning. Oil levels are good and I just recently had the transmission rebuilt so hoping that’s not the issue. I can’t seem to make the sound happen and haven’t been quick enough to get a video as it doesn’t last long and isn’t a constant sound. Any ideas what it could be? Sensor or something from putting the exhaust back on? It’s a 2012 Scion tC.