The most common reason why an invisible fence transmitter unit will be beeping is that there is a break somewhere in the wire. While there are some modern units that will actually inform you about this through text, a simple alarm is still the most common way to alert a fence owner. It's a good idea to familiarize yourself with the owner's manual of your invisible fence, though, as some units have multiple types of alarms, and knowing exactly what sounds your transmitter is making or what color lights are flashing can tell you a fair bit about what problem you are facing.