high monotonous tone in the background. (About 150 hertz)
150 Hz is actually fairly low, my (pure guess) is that it could be 400Hertz which would be equivalent to 50/60Hz hum in domestic power sound gear.
For reference middle C on piano is about 262 Hz.
Google 'middle c' and you will find a link to a site that will play it and other notes in case you don't have perfect pitch
And as to why UAL more than others, perhaps others don't use hand mike above 10,000 and it is more likely to pick up the power 'hum', again pure speculation.