That is effing weird. I don't think it's exhaust. Sounds like AC system venting or maybe evap system. Any codes? Does it make a difference if AC is on or off? Next time you make it do it, put your hand on low side AC line and see if it feels like's its coming from that system. Also check evap lines for leaks (which would likely have already set a code). Maybe listen around throttle body and air inlet tube as well.