The council have received several complaints of noise pollution after a strange buzzing and humming was heard across Leeds last night.
Social media was flooded with theories relating to the source of the din, which was reported across several postcodes in the north and east of the city.
'Mysterious noise' in east Leeds confirmed as alarm test by factory
Members of the public claimed it began in the evening and did not end until the early hours of the morning.
Facebook users speculated that the hum, which varied in pitch, could have originated from a number of sites, including the Energy Recovery Plant incinerator facility in Cross Green, the Knostrop sewage treatment works off Pontefract Lane, or be caused by work on railway lines in the Osmondthorpe area.
Leeds City Council's noise pollution team went out to investigate after receiving several calls from the public, but were unable to establish the source. However they stated that they did not believe it to have come from the LERF incinerator, and are continuing to look into the matter.