If it is not police or fire emergency vehicles that you hear, it is probably the village emergency sirens. The emergency sirens are tested on the first Tuesday of the month at 10:00 am. This is the same siren that you will hear for a tornado warning.
The sirens are activated by the police department. Remember, if you hear the sirens at any time other than at 10:00 am on the first Tuesday of the month, it means that you should immediately seek shelter. Vernon Hills residents should also keep in mind that the Village DOES NOT give an "all clear" signal. The best thing to do is to monitor local radio and television stations.
One more thing... If possible, please try to avoid calling the police to inquire about the sirens. This will be an extremely busy time for members of the department and they will be able to tell you little more than you will already know or can learn through the monitoring of local radio and televison stations.