Short-finned pilot whale grunts
Own research showed (...here...) that grunts represent 3% of all recorded vocalizations. Though grunts subjectively sound tonal, they seem to be sounds exclusively generated by pulses at high repetition rates (290-690 pulses per second) having broadband frequency distributions as well as several relatively narrowband frequencies with more energy. Grunts can often be observed during socializing behavior when group members are in visual sight and strongly interact with each other. Thus, it is believed that grunts are social sounds.