Test events called heartbeat events are automatically generated. You can also manually generate test alerts.
In order to confirm that the system is operating correctly, test events are automatically sent every day, one event from each node in your cluster. These are referred to as heartbeat events and are reported to an event group named Heartbeat Event."

When the node responds it is a sign that it is working and the event is resolved. It is common to ask for these to be removed, as there are other alerts that would notify if there were issues with connectivity, (ie: node offline or internal network link down). We have two knowledge base articles that work with how to remove from the alert or increase the duration between these events.

https://support.emc.com/kb/490436 - How to stop receiving email notification for HeartBeat event
https://support.emc.com/kb/493496 - 8.0 Daily Heartbeat Alert and lowering frequency of it

The easiest method in the above KB is to extend the duration between alerts to monthly with the following command:

# isi event settings modify --heartbeat-interval=monthly