It could be possible a remote machine is trying to access the host. If you are using a monitoring tool such as vCenter, enable a lockdown of the host so that you can only access the host via vCenter portal or the local host. Below is the steps to do so in vCenter 6.5: 

1. Select the host 

2. Click on Configure

3. Under System, select Security Profile

4. Under Lockdown Mode, select Edit

5. Choose Normal and ok