2016.12.5: web.psy/gremlin:
. when hearing ventilation systems or motors
it sounds like there are voices from them.
. this reminded me of reports from air service during the war
when the voices of gremlins could often be heard
due to sound of motors and winds.
. it is true that during survival extremes
people may tend to hallucinate,
but gremlin noises may also come from
a bored mind finding patterns in random sounds
like when seeing animals or faces in clouds.

def gremlin:
"small imaginary creature blamed for mechanical failures,"
oral use in R.A.F. aviators' slang from Malta, the Middle East and India
is said to date to 1923.
. popularized in World War II.
Of unknown origin. perhaps
Old English gremman "to anger, vex" + the -lin of goblin;
or Irish gruaimin "bad-tempered little fellow."

. "the gremlins were said to shout, giggle, whisper, growl,
or otherwise make noise so as to distract aircraft crews."
"One common trait in all reports is that through whatever means,
gremlins were known to be able to
adhere to the outer fuselage of planes
and to withstand incredible temperature extremes,
high altitudes, and violent winds."
"perhaps the period of the most intense alleged gremlin activity
was during the fierce fighting of World War II.
Reports of gremlins were especially prolific among
the UK’s RAF (Royal Air Force) units, especially the
high-altitude Photographic Reconnaissance Units (PRU),
which flew perilous missions ... at great heights
on photographic missions over enemy territory.
It was during these harrowing missions,
when pilots operated in bitter, biting cold ..."