A sickly buzzer sound occurs if they key is IN and the door is OPEN.
See also: Door switch
1972 USA Owners manual (Page 03 ... 03):
Your Datsun is equipped with an ignition key warning buzzer. A buzzer will sound when the key is left in the ignition switch and the driver's door is opened.

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USA Owner's Manual
Driving Safety and Comfort
KEY AND KEY WARNING BUZZER
Your Datsun is equipped with an ignition key warning buzzer system. A buzzer will sound when the key is left in the ignition switch and the driver's door is opened. This is your reminder to remove the key.
To shut off the buzzer, always turn the key to LOCK position to remove and lock all doors when leaving car unattended.