Issue – Event log is flooded with getting lots of EventID 1040 – Warnings –
The average of the most recent heartbeat intervals  for request [Sync] used by clients is less than or equal to .
Make sure that your firewall configuration is set to work correctly with Exchange ActiveSync and direct push technology. Specifically, make sure that your firewall is configured so that requests to Exchange ActiveSync do not expire before they have the opportunity to be processed.
Since the avarage is 470, changed the value of HeartbeatAlertThreshold from 540 to 400. in C:\program files\Microsoft\Exchange Server\v15/Client access\Sync\web.config file
recycled active sync apppool.