If you have heard it, you would think that it sounds like something between a frog croak’s, a chicken’s clucking and something else thrown in for good measure.
“The land hermit crab sometimes communicates by a whirring-chirping sound, though not too often. These noises usually have been heard during aggressive encounters in the crabs’ natural environment and are seldom heard in captivity. The only behavior observed that may resemble aggression has been when one crab shakes or rocks another to drive him from a shell the aggressor wants. Even in this act neither crab is harmed.” (Nash, P. 1976)
“Land hermit crabs… produce clicking sounds by intentionally tapping the shells and rapping legs as well as make stridulating sounds by rubbing legs together. This noise-making is used in hermit crab communication, including aggressive displays. If a hermit crab tries to climb onto another or makes contact with it, both crabs may stridulate in annoyance. (Philippe de Vosjoli, 1999)
de Vosjoli, Philippe. The Care of Land Hermit Crabs.Advanced Vivarium Systems; (August 1999)
Nash, P. Land Hermit Crabs by TFH Publications (1976)
In case you haven’t heard it before, there are a couple sites with audio clips.