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is it just me, or when someone is trying to sneak up on you when your reading, or on the computer, or somethin, and they dont make a sound, you suddenly need to turn around because you know someone is there.
i get this alot especialy when i’m reading a book, i mean, even with my cat, i’m sitting there doing something, suddenly i need to turn around, and i see my cat. is this some sort of subconsious instinct that our ancestors needed to survive, or is it just your subconcious reacting to sounds that you dont consiously hear. -
whenever the janitor at my school comes down the hall way, you can hear a high pitched squeal
i know its from his hearing aid, i hear that a lot from my grandma
what causes this, why cant the people that wear the hearing aid hear it, can it be adjusted so the aid works fine, but the squeal isint heard?
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WAG you are reacting to sounds that you don’t conciously hear, as you said, or maybe also you felt vibrations from the floor moving.
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This happens with microphones in PA systems too, or pretty much anywhere where amplified sound finds a path back into the amplifying device. It’s called feedback. The sound keeps getting amplified over and over in a big loop, and creates kind of a runaway situation where the amplifier keeps boosting the same signal over and over. The sound gets louder and louder until it reaches the limit of the amplifier, at which point it just kind of settles out and stays constant in the high pitch sequeal you hear. The beatles were the first to use this effect in a song, if I recall correctly.
People with hearing damage tend to lose high and low frequencies first, so it is possible your janitor can’t hear the squeal. I’ve known other people with hearing aids who have had the same problem, and they can hear it, but for them it’s a choice between turning the hearing aid down to the point where they can’t hear anything anyway or living with the squeal, or constantly adjusting the hearing aid’s gain to try and get it just under the level where feedback occurs.
I think there was a recent thread somewhere around here asking about a sixth sense of balance. The gist on balance is that you pick up and interpret senses from various (real) senses and react subconciously.
Seeing subtle changes in light, sounds, and termperature or a gentle movement of air, I’d WAG, works in the same way, which would be why hearing a slight breeze behind you that moves a curtain a little bit and changes a shadow in the corner of your eye a centimeter makes you leap out of your wits sometimes - and scares you more when you notice that there is no one there. 
I’d presume subtle temperature, smell, etc changes that we never notice tell us a lot that we don’t see as obviously as visual interpretation. After all, animals are great at knowing someone is walking towards them around a corner. Next time you are in the middle of a forest at night, notice what makes you looking over your shoulder.
Hearing aids - my grandfather can’t hear his hearing aid when it starts making that noise. We just tell him “Grandfather, you’re squeaking again” and he turns it down.