Uwan

An nihongo|"uwan"|うわん is a disembodied voice that inhabits old, abandoned temples and homes. According to ancient legends from Aomori Prefecture, when a person enters a one of these buildings, the formless "yōkai" shouts out an ear-piercing “Uwan!” The voice is only audible to people inside the building — those outside hear nothing. Since an uwan does not physically exist and only consists of sound, it poses no physical danger.

Ancient Japanese legends provide several examples of formless "yōkai" like the uwan, which consist of nothing but sound, light or other natural phenomena. In the Edo period, however, these demon spirits assumed physical bodies as artists such as Sawaki Suushi incorporated them into their work.

References

* http://www.pinktentacle.com/2008/02/edo-period-monster-paintings-by-sawaki-suushi/