Has anyone noticed something like this: several times when at a stop, I have noticed a prominent low-frequency rumbling noise from the from of the car. It has lasted for maybe 15 seconds each time. My guess is that it something in the HVAC system. I think it has occurred with higher fan speeds, but I haven't been able to make it happening by ramping up the fan. So far just a curiosity, but I'm hoping that it doesn't develop into a bigger irritant.