I experienced this once late last week. I don't know what the cause is - it's been pretty consistently in the 30's (I know thats a heat wave for some of you, but in Texas, it's COLD!) so there weren't any significant changes in the temps. When it happened, I had been driving my vehicle for about 40 minutes, so it's not like I'd just jumped into a cold vehicle and then heard it. My Civic used to do this periodically, but never as loud as the Nox was the other day. Not only was it loud, it happened about 4 times in the space of about 5 minutes - I was a bit unnerved by it, but thankfully haven't heard it again since then.