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Spurious character typed on console every ~20 secs until logged in

This has happened a couple of times or so in the past week:
On booting up, I get to the prompt to unlock my /home partition.
The first attempt fails, not because I mistyped the passphrase,
but because an extra ^@ character¹ was typed by the system.

If you now just watch and wait, an asterisk appears about every
20 seconds. The only way in is to start typing as soon as an asterisk
appears (starting with rubouts) because that gives enough time
to type the passphrase.

At the login prompt, ^@ will now appear every 20 seconds. If one
occurs while typing the username, it doesn't spoil the entry.
I haven't tested whether it would affect the password.

Once logged in, the phenomenon ceases and everything is normal.

There is a second peculiarity that also occurs on this system.
It happens at every boot, and has no effect on the proceedings
because it only *outputs* to the console rather than inputting.
Not long after booting the cursor jumps to the left side of
the line. If it happens during passphrase entry, it has no
effect: the partition unlocks as normal.
Having got to the login prompt, the same thing will happen
there, again with no effect. Again, it stops happening after
logging in. I've always just ignored it.

Has anyone heard of either of these happenings or know what
causes them.

The system is stretch running on a Lenovo ideapad U430 touch
in legacy mode (ie not the Windows/UEFI mode).

¹ not that you can tell what character is being typed yet,
because all characters reflect as asterisks at this stage.