Hello Mi_lad,
It sounds like you encountered some unexpected noise from your iPhone. We will be happy to look into this with you.
Do you happen to know which apps were open at the time this occurred? Have you closed any apps since this happened? You mentioned that you restarted the device, has the issue reoccurred after?
When the sound went off, did you see any visual notifications on the screen at all? If not, then let's go to Settings > Sounds & Haptics and look under ‘Sounds and vibration patterns.’ You may be able to identify what is making this noise by tapping on the items there. If you can't locate it there, go to Settings > Notifications. Apps that use sounds for alerts will be listed here. You may also be able to identify any apps causing this here as well.
You can read more about this here:
Use tones and ringtones with your iPhone, iPad, or iPod touch
Cheers!