My daughter has a 2013 Mazda 2 Touring. She said that 3 times over the last week when starting the car she will hear a single whooping sound that lasts about 3 seconds and then it goes away. Only happens on start up. Not sure if it happens with a cold engine vs normal operating temp. Otherwise the car runs fine. No engine codes present. No check engine light. Engine coolant is full. I haven't bee able to reproduce the sound. Maybe a vacuum connection going bad? Any ideas? Thanks to anyone who replies.

 

The whooping noise has yet to rear it's head today. I wonder what it could be. Maybe it's a ghost in the machine.