I live in Northern Canada and we regularly get -30°C weather. Lately my car has been making a rumbling noise at low RPMs and now sometimes while idling as well. It wasn't as noticeable at first, but it definitely is now. It's LOUD (sounds like a diesel truck). When I accelerate it will kick in for a brief second as I pass whatever RPM seems to be causing it (which seems to switch around a lot). Does anyone know what might be causing this?
I had that happen on a grand Cherokee I used to own and turned out when I had some warranty work done which involved removing the exhaust, they dropped it and broke the catalytic converter internally. Sounded just like a diesel. It was pieces of the ceramic catalyst honeycomb getting tossed around at certain engine rpms. I thought it was the muffler but my cousin at the muffler shop instantly said it was the cat. No check engine light either.