Prevent "Client null sending PINGREQ PINGRESP" messages

I last used Mosquitto on a Raspberry Pi running Stretch.

I am attempting to implement on the latest Raspberry Pi OS using the mosquitto_sub/pub command.

This works but I keep getting lots spurious output.

I tried --quiet but this didn’t seem to make any difference.

Ultimately I plan to send messages every 15 minutes.

How can I get a simple settup which just shows messages.