Try disconnecting the HDMI and compare your results.
If it were me, I would review my
/etc/cron.d/* scripts to see if any of those are set for the 120 second timing.
Next, I would try this in Safe Mode to see if the condition goes away. If so, then one of your plugins has a 120-second timer event.
If it's still there in Safe Mode then temporarily disable the Pi Support plugin's check via the Settings screen and test again. Here, you'd want to watch this with something like
watch cat /var/log/syslog | grep -i undervoltage perhaps, noting that the bundled Pi Support plugin is the one responsible for observing undervoltage conditions within OctoPrint.
Pi Support check for undervoltage itself is here at either 30 or 300 seconds.
There's also a network connectivity check which occurs at some frequency. You might try toggling that off to see if this has a positive effect.
You might also try toggling off SD card support in the Files side panel widget via the gear icon. (There's another timer which presumably checks for changes in its directory.)
I think there's a heartbeat timer at 900 seconds so this probably isn't your culprit.
My gut tells me that there's something writing to your microSD card at this interval. It could be within OctoPrint's space or not. For example, something could be writing to a global log in
And finally, disconnect your serial cable to your printer and re-test. If your Pi is sinking power over to your printer then this could be the cause.