Thank you so much for your suggestions as always!
It took less than I expected - I started killing process after process and it soon turned out it was a Docker container, specifically Pi Alert (jokobsk/pi.alert:latest).
I killed the Docker container in the middle of a PTR flood and the queries stopped immediately.
I had installed this container about a year ago and never really used it since. No idea why, but a couple of days ago it started sending out local PTR queries every other minute. I may investigate further when I have the time.