NANO resetting when connected to USB and Monitor not enabled

When my shield is connected to the USB port and the serial monitor is NOT enabled the sketch resets about every three seconds. I know this because I have a scan counter displayed on an OLED.
If I enable the serial monitor or power the shield using an external USB power supply the sketch does not reset.
I am using a Nano.
The port is COM1
IDE version 1.8.13
Processor selected : ATmega328
Windows 7 Professi0onal Service pack 1, 64 bit
The protective foam is removed.
I tried to set the board in Bootloader mode. Same problem.
This happens no matter what sketch I run.
It happens with “bareminimun” and “blink”.
Do you have any ideas?

1: What shield are you using?
2: Does this sketch (or a previously loaded one) use the wachdog timer?

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