An old config from a vcenter agent must still be there in the esxi, as please see the fact there : Required ports for VMware vCenter Server 5.5 (2051575)
This is the default port used by the vCenter Server system to send
data to managed hosts. Managed hosts also send a regular heartbeat
over UDP port 902 to the vCenter Server system. This port must not be
blocked by firewalls between the server and the hosts or between
Please follow that KB to remove any trace from Vcenter :
Uninstalling vpxa in ESXi (1005482)
To uninstall the vpxa agent:
1.From the main yellow and gray screen on the ESXi host, press Alt-F1.
2.At the next screen, type:
3.When prompted, enter the root password.
4.Execute the following command:
Note: The vpxa agent automatically re-installs once the ESXi host is
connected to vCenter Server.