hi all!! sorry if my English is not good.. my car is mazda3 1.6 sedan model 2010. the trouble is that when I put the key in the ignition and turn to ON position, there was kind of faint sound inside the engine bay (even the engine is stop, I also turn off A/C, radio).. I leave it for 10 minutes but it still there. when I turn key into ACC or LOCK, it gone. I dont know exactly where does it come from and hard to describe what does it sound, so I have a video if it helps you guys to figure it out.. I make sure the ambient sound is quiet when I record this video, the sound you will hear in the video is what I am talking about. please help and thank you!!
tieng keu la mazda3 2010 - YouTube

 

it happens when you turn your key to the on position because you just activated the electrical system. the faint sound you hear is most likely the electrical system humming/buzzing slightly. nothing to be concerned about. if you're too worried about it, take it to a garage and have them take a listen.