After some more research, it seems to be this may most probably be the Active Sense midi message:
From reading some online forums, it seems that Roland equipment adheres strictly to the midi protocol and that protocol says that a device should send Active Sense messages every 300ms
https://midi.org/summary-of-midi-1-0-messages
Most other manufactures do not send this message.
Roland have, for some equipment, started allowing users to disable sending this message:
I will create a Feature Request to:
- show Active Sense messages in the midi monitor
- allow to filter out Active Sense messages