Hello all I think I am here to give you the answer you have all been searching for. If you notice the sound usually right when you start the car in the morning, and it kinda sounds like a low rumbling sound like it has a big exhaust on it. I think your sound is the air pump pumping fresh air into the exhaust system. It sucks in fresh air and puts it into the exhaust for the emissions to help with the catalyst I think. But next time you hear the noise pop the hood and look under the battery shelf and listen to the air pump, it will get kinda gurgully sounding sometimes when they have water in them and they will set a check engine light.