In commercial songs, I sometimes notice they contain a faint "beep track". I'm wondering what the purpose is of the "beep track"? What is that purpose? Two songs that I know use them off the top of my head are Robert Miles - Fable and Evanescence - My Immortal (this one's really obvious). I've even seen them used in MIDIs.

What you're hearing is probably the click track - it's a MIDI time pulse to help the musicians keep time. If you're hearing it on the final track, someone really messed up during the recording process. Most likely: the vocalist had leaking headphones and the clicktrack was too loud in them, or the vocalist was a prima-donna who didn't want to wear headphones during tracking.

Hmmn - I'm not sure you're right MK.

I'll cut it out and host it later if anyone needs it, but you can really hear it in the last 5 seconds or so of "My Immortal", during the fadeout. It's most definitely not a click-track. It seems to be intentional (as it doesn’t fade out with everything else).

It also doesn’t explain why some MIDIs actually have the same thing. I could probably find a non-recorded song with one if I tried.

Also, what I'm referring to certainly isn't synched to the beat.

This is the last few second of My Immortal (OGG, 0.3meg). You can clearly hear the "beep track".

Other ones I've heard are less morse-code-ish (speaking of which, perhaps it's a coded message?). I'll try and find more later.

Word has it that what's in my immortal is caused by a synth playing up on sustained notes and they decided to leave it in there.

Now I'm thinking that it's effects and synths misbehaving that some very dedicated MIDI authors have decided to duplicate with instruments.

And here I am thinking it's some kind of clever mastering trick or something...

I have a better idea: it sounds like Morse code. If you're anything but as lazy as me, you can try to decipher it and see if it means anything. It doesn't sound like a "mistake" to me - it's not a delay, it's not a highpassed instrument and it's not a vocal. Furthermore, positioning says it's probably in a separate track, which makes it quite intentional. I say it's an "easter egg" (call it a subtle, but integral part of the song), although I don't even know who the author of the song is...

I'm with Crisp - morse code was the first thing that popped in to my head when I heard that short clip.

Do a tight bandpass around 2637 Hz and you'll hear it a lot better...

I'd just call it shitty production on the synth side. :\ Poor production can result in the most random sounds in music, even though a lot of the times you can hardly notice them.