You're talking on the phone with your friend and you hear a loud noise from his phone that sounds like a police car. You say this about the noise.
I hear some kind of siren in the background.
When you're listening to a telephone call, a radio show, or watching a video, sounds that are not close to the microphone are "in the background":
Listen - you can hear Rachel saying something in the background.
You can also see things in the background in a photo, a painting, or a video.
See that mountain in the background? That's Mt. Mitchell.
Sounds or sights that you see in real life (not on a phone call, in a movie, in a photo, etc.) are usually not described as being "in the background". Instead, you describe them as being "in the distance", "in the other room", and so on:
I hear some people talking outside.
Sirens are devices that make a loud noise to warn people of things. Police cars, fire trucks, and ambulances have sirens. Military bases and fire stations might also have sirens.
The fire alarms that people have in their homes are usually not called "sirens". A siren has to have a sound similar to a police truck or ambulance.
The phrase "some kind of ___" can be used when you don't know exactly what you're talking about or you don't want to specify:
What's this? Some kind of stew?
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