What's this sound? - Dogfight between F14 & MiG23
That is the aural conversion of the input of an AIM-9 missile's infrared seeker head.  The missile allows the shooter to operate it in either a caged mode, that is where the seeker head is slaved to the angle and azimuth of the radar antenna, or uncaged mode, where the seeker head can freely seek out and track a bright infrared source like a jet engine on a fighter.

When the AIM-9 missile made its debut, the aural lock on cues were the only means a pilot had to know that the missile had acquired the target.  Today, pilots can often select this engagement mode if they wish to keep their fire control radars off in order to avoid detection.  When the missile seeker head is searching for a target, it makes a mechanical, raspy growling sound.  Once the seeker head acquires a heat source, the tempo of this growl picks up in conjunction with a steady mid to high-pitch tone.  You will often hear pilots acquiring a lock on an enemy aircraft refer to it as "getting a tone"; this is what they are talking about.