Taking a stab in the dark.. the unit is over heating...?
LOL. yeah it sounds to me that the computer is over heating. My comp is suppose to make a siren sound when it overheats (but i have never witnessed it). Anyways, i would first check to make sure that there arent any funky alarms set on the computer. (maybe his mom doesnt want him to play for over 30 minutes so it makes a siren). Also, what applications do you run when it starts to make the alarm sound? I dont know what the problem is, but because it is summer time and because it is hot i think that the computer is overheating. Take it in, replace the fan(s) and get your hardware checked to make sure it doesnt fry.