What you observed is possibly the same phenomenon I get sometimes from the TV. A high pitched sound at the upper limit of hearing gets the ear confused by trying to listen hard at that frequency and other, lower frequency, sounds get partially ignored.
At least that's my theory, maybe the boffins have a better explanation.
I very much doubt that one could do that deliberatly without some damage to the ear. I mean, who wants to listen to a high pitched noise in order to miss out on the low pitched noises?
You can however, to some extend, cancel noise by wearing a headphone where the sound you want to hear comes from one source and all unwanted noise is picked up by a second microphone and is fed to the headphone 180 degrees out of phase. The sort of head sets chopper pilots wear to cut the engine noise.
Not a good idea on the road as you need to hear the siren of the ambulance behind you, trying to pass.
Klaus