Avalanche beacons transmit a beep followed by a period of silence. The receiving beacon uses the strength of the "beep-beep-beep" signal to locate the buried transceiver. The period between the beeps should be silent.
Most older analog beacons (e.g., the Ortovox F1 and F2) and some digital beacons (e.g., the Ortovox X1 and early versions of the Ortovox D3) transmit a weak signal during what should be the "silent" period. This continuous background tone is sometimes referred to as "continuous carrier." The red line in the following picture illustrates this continuous background signal.
Of course, the receiving beacon always receives some ambient background "noise," but the continuous signal described here is actually transmitted by the transceiver.
If you have an analog transceiver, you can test your avalanche beacon to see if it is transmitting a continuous background signal:
The background signal is not a huge problem with a single burial, but it can cause major problems for digital transceivers during a multiple burial, because the background signal will mask the other transmitters (read about multiple burials on AvyRescue.com). This is most obvious when the transceiver that is emitting a continuous carrier is closer to the searcher than the second transmitter. A continuous carrier can also cause digital beacons to mistakenly report that there is more than one transmitter (i.e., "a ghost" transceiver).