PEP modem such as the Telebit Trailblazer as it tries to synchronize with another PEP modem for their special high-speed mode. This sound isn't anything like the normal two-tone handshake between conventional modems and is instantly recognizable to anyone who has heard it more than once. It sounds, in fact, very much like whale songs. This noise is also called "the moose call" or "moose tones".