All About the Harbor Seal - Communication

Harbor seals are probably the least vocal of all pinnipeds.
In air, they may snort, hiss, growl, or sneeze - often as a threat to another seal. Harbor seals vocalize mainly underwater.
Pups vocalize more frequently than adults, especially with their mothers. Pups' sheep-like cries are individually distinctive to their mothers.
Mature males vocalize underwater as part of a display during the breeding season.

A harbor seal may slap the water or its own body with a pectoral flipper to show aggression, or during courtship.