RT high tone carrier wave on UAL?
 

During the last couple of years while flying, I noticed that all transmissions of United airlines have a distinct carrier wave. Some sort of high monotonous tone in the background. (About 150 hertz).
Why is that? And why only with that airline?
I must say that in the airspace that i work in, all ual flights are wide bodies. (Europe)
If someone knows, i would be delighted.

They use the hand mic above 10,000 feet. This is the background noise you are hearing.

I know what you mean but it isn't just UAL.

Always presumed it was a 757/767 thing.

FWIW same thing can be heard for announcements in the cabin.

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.

SOP - boom mikes required below 18,000'. "Most" crews won't go on speaker/hand mike until cruise. Flying over the pond, virtually all crews will be on speaker/handmike.

I thought it was the humming of the chemtrails machine.