You will have to try to narrow it do you might try booting in to safe mode to see if it is from an App you installed or from an installed Google App. Booting into safe mode will disable all installed Apps, then see if it still dose it.
Do you have your phone in a case that also hold your bank card (or similar chipped swipe card)? If so try turning off NFC. Mine would make a constant bell sound when ever I woke the phone up or moved the case flap (holding my cards) away from the phone and back again. Was the NFC trying to interact with my bank card.