Nano flashing LED13 by itself?

Hi folks,
I've just swapped my project from a UNO to a NANO, and now have a weird problem of it flashing the built in LED by itself.
Once per minute it will flash twice then 5 times. Very regularly after ever minute. No dim glow or random weirdness. This is programmed somewhere.
I loaded up the Blink sketch, then commented out the loop commands, even wrote pin 13 LOW in the set up. But it still does it.

Would it be something in the bootloader that is doing this? and if so - why?

Many thanks for your help.

What type of Nano? There are several Arduino boards that have the "Nano" name these days, and they are quite different to each other and different to the classic Nano 3.

Do you have another Nano to test?

Does the flashing happen if the Nano is powered by batteries or an external PSU, instead of through usb? Are you certain it is the pin 13 led that is flashing, and not the tx/Rx LEDs? If you run the blink sketch, using pin 13, do you still get the unexpected flashing on addition to the regular flashing?

Load a blank sketch. Post a pic of the flashing lights with a blank sketch loaded.

Yes it is the onboard led that is flashing.
No the RX and TX are not lit.
Yes the problem persists when running the blink sketch and interferes with it, then blink resumes for the next minute and repeats.

BUT it seems it does not do this when on battery power.
Looks like it is related to being plugged into the laptop.

Oh dear.
I have just tried it out on battery power and it worked fine with no problem.
Now I have it plugged back into the laptop the problem has disappeared.

This means I am unlikely to troubleshoot further or find the reason for it., but I will come back here if it persists.
Thank you for the suggestion to try it on battery power as that identified the cause I think.